Hello;
I read your post again and thought about cheating. Yes, cheating data. Say: you set your equipment to sent seven data bits + your receive bit. The HP48 will read eight bits, but you know one of them is your golden bit.
I do not have the 48's User Guide in hands, but I am almost sure you can set how many data bits as you can have in receiving/sending data.
Just a suggestion.
Cheers.