What you suggest by tacking fields onto the end probably doesn't work because of mIRC's support for filenames having spaces. Because the "file name" can potentially use more than 1 token if the filename has spaces in it and the 'fill spaces' option is unchecked. A script probably should be finding the filename by parsing for a token ending with a quote, but this works with the existing protocol too, and is much faster:
//var %a DCC RESUME "name with spaces.jpg" 12345 654321 | echo -a port $gettok(%a,-2,32) offset $gettok(%a,-1,32) filename $gettok(%a,3--3,32)
Scripts will be broken if you tack on extra fields, and I don't know how older/existing clients would behave if they encountered unknown fields.