Something like this? Example used for the OPTIONS command.
Although I'm not certain it will be able to capture the commands of your program.
(vl-load-com)
(setq VTFRXN (vlr-editor-reactor nil '((:VLR-commandEnded . VTF))))
(defun VTF (CALL CALLBACK)
(if (= (strcase (car CALLBACK)) "OPTIONS")
(progn
(setq info (getvar 'MODEMACRO)
info (vl-string-left-trim " /SPI-:1234567890" info)
info (strcat "SPI- 1:50 / " info)
)
(setvar 'MODEMACRO info)
)
)
)
If this doesn't work you probably will need to create some test reactors for different reactor types just to see what reactors your program command triggers and what not. But be careful, it is really easy (as you have noticed with the SysVar reactor) to crash AutoCAD, so make sure to save your code before each test.