I think you’d have to do what @Morgan_Hardwood suggest and use “$@”. From his link:
“$@” - Expands to all the words of all the positional parameters. Double quoted, it expands to a list of them all as individual words.
That might be able to be passed to montage directly that way. So replacing P12801{00..90}.jpg with "$@" should expand what is passed - then work the rest of the way as indicated by @Morgan_Hardwood.