Wasn't aware that %@execstr could call BTM files. However this seems to be slower than using the alias approach (@Rex: could you comment on this?). Also, this approach suffers from the below problem too.
Unfortunately setting PROMPT in tcstart.btm seems to be treated differently. Setting PROMPT from the command line (e.g. set prompt=%_PID%$s%%git_prompt%%$g$s
) works (set PROMPT gives me: <some_pid>$s%git_prompt%$g$s
); setting PROMPT in tcstart.btm (same assignment) doesn't work (set PROMPT gives me: <some_pid>$s$g$s
), the variable gets eliminated.
I also tried `%...%`, ^%...^%,`%%...%%`, and ^%^%...^%^%, to no avail.