Coupla random thoughts/observations:
1) Dorsiflexion is neither innate (go check out a baby's ROM), nor largely determined by bone architecture. More about ligaments and tendons. Typically we lose range of motion in same as we age. Had PT a while back for ankle issues, guy said,"Oh, you don't have enough dorsiflexion, that's causing stress on your joint," and proceeded to stretch out things, give me some exercises. Felt odd for a while, but have discovered it takes less muscle to do retraction turns, since I'm not fighting my own stretch reflex when I pressure the boot.
2) I'd be dubious about modifying existing boots for significantly more upright stance unless they're designed for variable stance. Would guess you can remove material from the rear cuff in any boot, but I always thought the spine is integral to how the the boot transmits force laterally, no? I've never had great success with Frankenboots, but then my German accent is bad. This may be a spiffy excuse to buy new boots.
3) I've found one advantage to skiing in a more upright position, less forward lean, is that it allows more pronounced sinking or rising movements if you need them. For instance in bumps. If you're already forward and halfway down, eg., "compact," less range left, things get stiffer. Had an instructor work with me on this last spring, when all we had were bumps. Or mud.