can you send the SVee6 a TSIP "sentence" to turn on extra NMEA sentences

The docs imply you can.

http://www.dementia.org/~shadow/sv8plus_manual.pdf describes the protocol. I have actual SVee6 docs somewhere but the only difference is the hardware description; The SVee8 has 2 more channels and another serial port.

Why not not have a configuration setting [GPS menu option],
Well, it does mean that when you change GPSs if you forget you're screwed, I'd guess.

Or does it need to be sent each boot up?
Depends on the GPS. That I wasn't clear on, for the SVee6, like, does it forget when I shut off my car? I'd guess no, that it's more or less the same as the NMEA/TSIP/TAIP selection. An Earthmate or a Tripmate *will* forget. The Magellan Gold I have doesn't have a way I've found yet to configure it other than from the menu on the screen of the unit. And so on.