BeoMegaMan:
I guess I may be a dying breed with an old classic as the BC7700.
Certainly not! But, I guess the intersection of the vintage / modern stuff enthusiasts is rather small after all - so you may well be the first one to even think about this problem.
I don't personally have any of this kit, but I tried to work out the BC7700 service manual. It's not complete with the parts lists and the internals of the remotes don't show much detail. Anyway, both the MCP and the Beocenter have a crystal oscillator working at 400 kHz. In the Beocenter, the output of that goes through an on/off control gate and a 1:10 divider. So, I'm pretty sure the IR carrier frequency from the Beocenter to the MCP is 40 kHz, and so there's a good chance Beo5 could work it.
There might be a catch with the carrier from the remotes to the Beocenter - which you actually need - being different. The schematic doesn't help here, as the receiver uses a TDA4060 preamp IC, which will work anywhere between 0-100 kHz. There's a tunable LC circuit before it taking care of the selectivity, but there's no mention of the coil value so I can't calculate its usable range.
Enough with the technical gibberish - in conclusion I think this would be well worth attempting But, with this documentation there's no way getting around that you need first some other way of capturing the codes.