This has been discussed and it seems like the most realistic way is to use the Paster plugin - load the full sprite strip into a full-sized regular sprite, calculate (or know) the size of a single frame - place a Paster object of the correct size over the main object and paste that onto the paster object. Now you have a single frame of the correct size. Load that using imageurl into your main object, move the spriter to the next position, repeat.
Kinda like that. I'll be very glad if someone comes up with a more elegant solution.