Since the .50 Beowulf is just a caliber loaded to the standard AR platform, I would say NO, you can't own one on an FID. No magazine modification is needed to use an AR 30 round which holds 12 Beowulf easily. (Although you will want to smooth the lips or you'll gouge the casings.) At this time, I do not know any other platform for this round. Also, the 90 round aftermarket AR mag can hold up to 32 rounds with no modifications.
Definitely need an LTC for the above reasons. If age is your limitation for not having a LTC; have patience. It will be worth it. The Beowulf is very fun to shoot. Hopefully ammo availability gets worked out soon.