Q:

Is there a tradition to pronounce kamatz katan as "a"? 

There are many ways of pronouncing Hebrew in prayers and when reading the Tora.  Most of these are tradition-based: one pronounces it the same way his parent or teacher did, or a close approximation.  Over the years, of course, the amount of error in those approximations amounts to quite a bit, which presumably explains the wide divergence of pronunciation. 
Ashk'nazim typically pronounce a kamatz as a fairly back, not very open vowel: something like ʌ or ɔ. 
S'faradim, on the other hand, typically pronounce

a kamatz katan as a fairly closed, back, round vowel, something like o; and
a kamatz gadol as a fairly open, central, unrounded vowel, something like a.

Now, I've heard some people pronounce even a kamatz katan as a fairly open, central, unrounded vowel, something like a. (Or many a kamatz katan, anyway.)  I've always assumed this is not based on the tradition they received from a line of teachers and ancestors: am I right, or is there some tradition that pronounces the vowel this way? 

A:

All Sephardic/Mizrahi (but not Yemenite) reading traditions distinguish qames qatan (pronounced as you point out approximately "o") from qames gadhol (pronounced as you point out approximately "a").   Most Ashkenazim, along with Yemenites, pronounce both approximately as "aw" in claw, draw.   Some Ashkenazim pronounce qames gadhol as "aw" and qames qatan as "o". 
Great confusion has resulted when people from an Ashkenazic background make a partial switch to Modern Hebrew pronunciation.   They often don't know to distinguish qames qatan from qames gadhol, and even if they do don't know which is which. 
One should note that the rules of qames qatan in Modern Hebrew differ from the Sephardic reading tradition as follows:  a qames preceding hateph-qames is qames-qatan according to Modern Hebrew (e.g., tsohorayim), but is qames gadhol according to Sephardic reading tradition (tsahorayim). 
It would be a reasonable assumption that where the last-mentioned rule does not apply, someone pronounced qames-qatan as "a" is simply making an error.  
There are, of course, many situations where authorities dispute over a particular qames followed by a shewa as to whether it is qames-qatan followed by shewa nah or qames-gadhol followed by shewa-na`. 