This raises an interesting question in my mind. Not that it would be a good regular operating practice, but is it even possible that a transmission could be started (say a final 73 message) and then at about 6.5 or 7 seconds in, a decode could be double-clicked to call that station, and BOTH messages would be decodable? Or is the minimum time required for a decodable transmission simply too long to allow for two? If length of text affects time required to transmit, then my short call might contribute to being theoretically able to combine two messages. My final 73 to a 4-character callsign would be just 12 characters, and a TX2 message would be just 13.

Hi Randy,
To put this on the right rails may I refer to an excellent paper https://physics.princeton.edu/pulsar/k1jt/FT4_FT8_QEX.pdf
It describes how the messages are constructed and source coded. The most relevant issue about callsigns is that the number of characters as such is not the issue, but there are strict rules what a standard callsign can contain.
For you purpose there is already a message available in the Table 1.
0.1 DXpedition K1ABC RR73; W9XYZ <KH1/KH7Z> -08
It is used for DXpedition protocol. The use it for normal operation would set more strict rules for a QSO. E.g. in the example message K1ABC needs to be satisfied to the reception of the RR73 as the indication of the end of the QSO and no 73 should be used. Otherwise there is a high probability that the sender of that message would have two independent QSOs at the same time one with K1ABC and second with KH1/KH7Z. Well that would already happen, when K1ABC does not receive the RR73!
73, Reino OH3mA