That will likely work as long as the path doesn't have any non-ascii characters. If the path has non-ascii characters the 'posix path' keywords will need to be added and accented characters will have to be represented with escaped octal byte codes. Luckily the Terminal's path completion system will do this for you--just press Tab while you're entering the path and the Terminal will complete it for you (if there are other folders that have similar names then the Terminal will beep at you until you either enter enough characters to differentiate your chosen path or press Tab again which will cause the Terminal to show you the matches).
Hmm, I just tested this on a directory with an ndash (–) and one with an o-umlaut (ö) and ampersand (&) in the name and it just worked…
Note, however that I work in a UTF-8 enabled shell (but to my knowledge that's the default on a Mac these days) but as this also works just fine in the applescript editor, I think non-ascii characters are actually not a problem (any longer?) — or am I doing something special here?
Instead of using multiple -e arguments to osascript you can also run it with a multi-line “script”:
osascript -e 'tell application "Photo Mechanic 5"
open "<full path to folder>"
end tell'(the use of the single quote saved me from having to escape the double quotes, use what you like in your script)