[Nokia Lumia 900] kenikh:
I've been analyzing the Finnish Tango ROM files from the download package and have noticed a few things:

Obviously, RM-823 is in all of the file names

When you crack open the various XML files, you'll see local path references to resource files

You'll also see GUIDs for digital signatures, like this one in "RM823_059N8W8_2175.1600.8773.12140_009_signature.bin":

<?xml version="1.0" encoding="UTF-8"?><Signatures
Version="2">
<Signature>
<File>[COLOR=royalblue]RM823[/COLOR]_[COLOR=seagreen]059N8W8[/COLOR]_2175.1600.8773.12140_009.vpl</File>
<DigitalSignature>[COLOR=red]8F6EA16B84B8E0A0BA42CCF4159DE183748DE4FCE359671C9C0FE9927FAF0200591945A185CA702FD04594DF254338D777877565EA1A4958D765EE558548001E38923A688F793C5373356843C6E27AB361CB5472083EE53514B03E9E5ED709CBD6705066A24865C7E5977105DA2D7EA037EBFC91CD1336B68DCF6EC36C095364[/COLOR]</DigitalSignature>
</Signature>
<Signature>
<File>[COLOR=royalblue]RM823[/COLOR]_[COLOR=seagreen]059N8W8[/COLOR]_2175.1600.8773.12140_009.dcp</File>
<DigitalSignature>[COLOR=red]7F1B8364441BD834DD7A90ECDEEAA846A32ACB46EBF3147A012A3E9F16B8DFA8680377686C5091900EBF5FAB617AB7EEF4491906CE7DD1AD37ECBBEAB16FB25DC1E96D4646CF8D12B0C0F3E408ECD89E02166A4251686F6C7F7943E841E11791F208F051FF26B804ED91692AE538D772BA2950FB4B2D996B836FD36ED56EC8A8[/COLOR]</DigitalSignature>
</Signature>
</Signatures>

Signature

Product Code

Model

I am guessing that a few different steps may help in getting this ROM flashed onto an RM-808 device:

Rename the files so "RM823" = "RM808"

Update the product codes to suit in the file name, as well

Update all paths in XML files to the new file names and product codes

Here's the stretch:

Locate all of the digital signature references in the XML files of the downloaded RM823 package and the latest RM808 packages for your phone.
​

Replace the Signatures in the RM823 files with the respective signature from the RM808 package.

While this may not work, it is at least a reasonable next step to try to get this working. Depending on how the Signature GUIDs are validated (e.g. if they are static to XML and not verified in compiled code), it could work.

If not, at least we'll know a little bit more about how these packages are structured.

FWIW, the .vpl file appears to be the key to coordinating the pieces (see image)

As you can see in the XML (reformatted for legibility):

<?xml version="1.0" encoding="UTF-8"?>

<VariantPackingList>

<Tracking>

<OriginalFileName>RM808_059M7R3_2175.1002.8112.12084_001.vpl</OriginalFileName>

<TimeStamp>2012-04-20T10:44:36Z</TimeStamp>

<Origin>Bart Datapackage Generator -
simonton</Origin>

</Tracking>

<Variant>
<VariantIdentification>
<Description>RM-808 VAR ATT WHITE GLOSS</Description>
<TypeDesignator>RM-808</TypeDesignator>
<ProductCode>059M7R3</ProductCode>
</VariantIdentification>

<SwVersion>2175.1002.8112.12084</SwVersion>
<VariantVersion>001</VariantVersion>

<FileList>

<File>
<Name>RM808_059M7R3_2175.1002.8112.12084_001.dcp</Name>
<FileType>Dcp</FileType>
<Signed>true</Signed>
<Optional>false</Optional>
<Crc>E92868AF</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_adsp.esco</Name>
<FileType>Esco</FileType>
<FileSubType>ModemSw</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>414BBFF8</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_amss.esco</Name>
<FileType>Esco</FileType>
<FileSubType>ModemSw</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>5B4DE746</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus.esco</Name>
<FileType>Esco</FileType>
<FileSubType>WindowsPhone</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>4772BE08</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_user_area_erase.esco</Name>
<FileType>Esco</FileType>
<FileSubType>UserAreaErase</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>98F7A27C</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_mdm_amss.esco</Name>
<FileType>Esco</FileType>
<FileSubType>ModemSw</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>22AAD657</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_mdm_dsp1.esco</Name>
<FileType>Esco</FileType>
<FileSubType>ModemSw</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>113DEA4D</Crc>
</File>

<File>
<Name>RM808_12w08_4_prod_attus_mdm_dsp2.esco</Name>
<FileType>Esco</FileType>
<FileSubType>ModemSw</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>CF02B420</Crc>
</File>

<File>
<Name>rm808_059M7R3_label.pcx</Name>
<FileType>Binary</FileType>
<FileSubType>TypeLabelPicture</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>C75AE395</Crc>
</File>

<File>
<Name>rm808_059M7R3_label.xml</Name>
<FileType>Xml</FileType>
<FileSubType>TypeLabelConfiguration</FileSubType>
<Signed>false</Signed>
<Optional>false</Optional>
<Crc>EECD5971</Crc>
</File>

<File>
<Name>RM808_059M7R3_2175.1002.8112.12084_001_signature.bin</Name>
<FileType>Signature</FileType>
<Optional>false</Optional>
</File>
</FileList>
</Variant>
</VariantPackingList>