Regardless of what is holding the balls in place, skewing the trucks during installation on the rails can drop balls. Even on nice big THK trucks. DAMHIKT.
For this reason, expecting to install the trucks as an assembly will not go very well.
THK and other have trucks with outer flanges for bolting the on from the bottom. If you are not designing for what you have one hand, then you can certainly get the trucks designed to meet your needs.
Not sure if Hiwin rails have that type of truck or not, but I would suspect they do.
If you use a little care when installing the trucks, there should be no issue.
Place some wraps of electrical tape around the other end of the rails. That may save a toe hit and ball chase.
Again, DAMHIKT.