The rendering algorithm is described in the Unicode book and I have implemented it in my arabjoin perl script. Skolt Sami still needs a few more accents. Note that RFC 1345 and GNU recode contain errors and use a preliminary and different latin6. It contains some combining vowel and tone marks that have to be written above or below the consonants. It is unlikely that there will ever be a Vietnamese part.
Some letters carry a combination of two different accents.
They are so many that they simply don't fit into the model of ISO-8859. It resembles WinBaltic . ISO 8859-15 will probably be implemented by a number of vendors, but it will take some time until a large percentage of the users start using those versions. We have just the place for ISO 8859-15 here in London. What a relief that we now have Unicode and won't have to implement this amusing piece of history.