“Fra” would be a goofy word to decline. You’d either have to use third declension or first declension. Just going with “frater” seems easier.
But I don’t think anyone is trying to say that fra is declinable!
After all, it is supposed to be an abbreviation of frater. The -a at the end of fra is not a termination like the final -a in puella, nor is it anything like the final -a in drama (appearing at the end because the word can’t end in -t)! It is just the a of frater appearing before the implied final -ter!
The use of fra as an abbreviation for frater has mostly (if not exclusively) been used in the vocative among the Neo-Latinist crowd, but that does not take away from the fact that abbreviations really aren’t the sorts of words that lend themselves to regular-word inflection, whether they end in friendly-to-Latin-declension letters or not. After all, abbreviations like SPQR and COS and PA and CA are not at all freely declined!
Fra would not be declined like drama, anyway, because words like drama, dramatis (stem dramat-) are Greek-derived neuter words, and fra is neither Greek-derived nor neuter. The problem with declining fra like puella is more subtle, though. While there are plenty of masculine words of the first declension (agricola, for instance), the a in fra is long, and words of the first declension regularly have a short a in the vocative and nominative. The a of fra would have to shorten for some reason. I suppose you can make the a of fra as the stem vowel of the word, but you know, at this point, that would be making things more complicated. I think one should either use the actual word frater as it is or treat the fra as the abbreviation that it is.
But what are we to do in Latin with a plural like the English “bros”? Perhaps, if fra should be used instead of frater, fra’es could be used and only with the understanding that it is just an abbreviation of fratres.