rhinoscript can:
StartupScriptCount
Returns the number of startup scripts.
StartupScriptList
Return the list of startup script files.
AddStartupScript
Adds a startup script to RhinoScript.
DeleteStartupScript
Removes a startup script to RhinoScript.
my guess is, if you add a file with .py this will also run.
You might also workarround by calling a command of a custom (your) (rhinocommon) plug-in.
rhinoscript is directly accessing the c++ sdk methods.
phyton and rhinoscriptsyntax are based on rhinocommon.
image from here
there might be difference of what is accessible.
i never tried, but you might also be able to add a startup script using a macro.
starting with
!-_documentproperties _options
because of this thread i think @stevebaer might be the right person to answer your question with more details / options / alternatives.