Added info:
I spent a few minutes researching this. The question comes up every few years, it seems. There are a few gunsmiths doing the conversions. Most are 7.62 conversions, but there are a few who managed to get a BAR magazine to work.
I found this link showing a functioning BAR magazine, by Shuffs Parkerizing -
http://www.youtube.com/watch?v=Fd2skyrYr_w
19 rds usable in a 20 rd magazine, and no bolt hold-open...