IMO transport in a WC is scary and the seat belt is the least of the worries.
1) The first rule of child restraint use is a seat that "will be used correctly EVERY time." WC tie downs are tricky to use. It's easy to miss a strap (guilty
), get one twisted or less than rock solid, or not get it anchored on the frame correctly. I was shocked to see one friend's chair "anchored" at the foot rest (a removeable piece) or very low on the frame after the official anchor points broke off (the chair base is 6 years old, so when you figure there are 4 trips per day for a "normal" school day, that's alot of wear and tear).
2) If there's lots of support on the WC (laterals, raised sides for hips/thighs), it's tricky to actually get the belt routed around the chair, much less keep it low on the hips/pelvis.
3) Power chairs are SUPER heavy so the thought of ANY crash while transporting in one of those scares me (probably because several of my young friends are power chair users).
Then I think about their siblings riding beside/behind the chair. 28" (or more because the bench in one van has center only LATCH/top tether for 2 kids in CRs) of head excursion = face plant into metal.
4) Most families have a minivan (mostly because of COST), so it is very tight quarters to maneuver the chair up the ramp and into position for tie downs. As the child grows, so does the chair, making it even harder (if not impossible) to have the WC ff ing, especially for families with backseat passengers.
Sure, you could customize the van so the child in the WC rides in the 'way back,' but then you loose ALL storage, can't hand them a sippy cup, book or snack. It's also harder to communicate, especially for kids with speech impairments or who use gestures or communication boards.
5) Then there's the tray issue. If you take it off, (you're supposed to) where do you put it so it isn't a projectile? In some cases, the tray helps provide upper body support or is how the child "holds" a cup or book, and/or supports the communication system. For some reason, the van customizers don't address this need.
Sorry. Rant over. After spending so much time discussing needs, researching and choosing very expensive equipment, waiting FOREVER for all the authorizations to be completed and the work actually done, its frustrating to still have safety concerns and significant mechanical, design or ease of use issues. And ALL of families I work with have had problems and concerns.