As far as I know, b) is not possible because the AX.25 frame format that APRS uses only allows for 6 bytes for the callsign and 1 byte for the SSID (and flag bits).
Personally, I think including the prefixed call sign in the comment field (Mic-E status text etc.) is good enough. You’re probably including your GPS position anyway, making it possible to identify which country you are transmitting from. OK, on second thought, that could be a bit difficult while you’re on top of OE/TI-035 or OE/VB-001
Interestingly, the Austrian “Amateurfunkverordnung” law frees Austrian hams from the usual requirement of suffixing their callsign with the visited district number or /P if they use APRS and also transmit their GPS position ( § 22 (2a)). However, it does not say anything about CEPT prefixes.
73, Manuel HB9DQM