Here we go, found it. It's mentioned in whatsnew.txt from back in the development of 1.08
[snip]
+ Added the ability to execute an MPL program instead of the command line
for a protocol. By starting your command line with a !, you can have
Mystic run a MPE program. For example:
send Command: !test %1 %2 %3
The above would execute test.mpe from your scripts directory and pass
the results of the %1 %2 %3 protocol MCI codes as command parameters to
the MPL program. Keep in mind that if you do use this to execute some
type of protocol, you must set the DSZLOG environment variable yourself
and have it point to the current node's temp directory as xfer.log. Mystic
will also create an xfer.bat or xfer.sh (depending on operating system) which
can also be executed.
[snip]
Kerr Avon [Blake's 7] 'I'm not expendable, I'm not stupid and I'm not going'
avon[at]bbs.nz | bbs.nz | fsxnet.nz
