Ok, I finally found the [hopefully] last bug of librfid's ISO 14443-4 PCD side tx side chaining bug. It only occurred in combination with WTX reception.
I also tested CID (card-id) support for multi-activation. If anyone has ever seen a 14443-4 PICC that actually offers NAD (node address) support, please do let me know (samples even more welcome).
To my surprise I discovered, that higher baudrates are actually already negotiated between PCD (reader) and PICC (transponder). Thought there was some practical problem, but it actually worked all the time.