So - yes it works. Yes I have confirmed it works. But the runtime is incomplete, so it probably is far from being a real contender. But I have 100% compiled an exe on a Mac under Catalina, ftp’d it to BeOS R5 running on a PowerMac clone, and run it. See this thread for more info. This includes linking to libroot.so and also libbe.so too iirc.
It is also worth noting, you can use any Metrowerks C++ compiler that supports PEF to build BeOS apps. I have used the one that comes with Codewarrior 10 (I think?) under MacOS X Panther to make a simple graphical BeOS app (see this link.) You basically just need the libs and the headers. I could probably send you the CodeWarrior target stationary/template I made - which just creates a basic BeApp skeleton.