In dev there's a Bela_initRtBackend() https://github.com/BelaPlatform/Bela/blob/dev/core/RtWrappers.cpp#L323 which can safely be called more than once from any context. However, it may be expensive because it uses sts::call_once() so it shouldn't be called from a real-time context. After it's been called at least once you should be able to use rt_printf(). The gXenomaiInited flag could be used for a quick lookup (it's not in Bela.h so you need to devlare it as extern).
This said there's normally no point in using rt_printf() from a constructor: constructors tend to do non realtime safe stuff and are often accompanied by memory allocation (unless you use something like emplace_back, I guess?) And so in general they are not real-time safe and therefore there should be little use in using rt_printf over printf.
