(just because I think the subject is interesting)
So much to cover here......
When it comes to using an FCC Callsign issued in one particular service.........
I would think that it would depend on whether or not a "callsign" is required in BOTH services. Like... I would not expect that it would be acceptable to use an amateur service callsign in the GMRS service. In my thinking.... Part 97 requires a valid callsign to be used in the allowed amateur bands that are specified by Part 97. Likewise, I would not think it acceptable to use the GMRS issued callsign in the Part 97 Amateur bands. I believe that this is because each of those two services states that there are requirements for licenses (GMRS is 95.1705 and Amateur is 97.5). Each of those two services also states a requirement for "Station Identification" (GMRS is 95.1751 and Amateur is 97.119).
CB - Part 95 Subpart D - does not say anything at all about either subject. It does not mention a license and it does not mention any "Station Identification" requirements. At all !!!! It doesn't say what you CAN use. It doesn't say what you CAN'T use. My interpretation here is that they "don't have a dog in the fight" regarding how one identifies themselves.... as there is not even a requirement that they DO identify themselves.
So, adapting to that, handles have been adopted... and the choice it left up to the individual operator. I've seen people using their names, organizational callsigns, self-assigned callsigns.... basically... whatever they want to call themselves is fine.
Using a callsign that was assigned for one service.... in another service where a callsign is required and is of a different "format specification".... does not satisfy the requirement of the callsign in that band. So essentially... the violation is that you are not using a callsign that is appropriate for that band.
But using a callsign that was assigned for any service.... in a service that does NOT require a callsign or even identification.... I'm not sure that I think it is illegal.... and to that end.... I would possibly scratch my head, think about it for a minute....and then move on... and struggle to live the rest of a satisfying and meaningful life. I don't think any ill effects would persist!!!!!