I googled “replacement Velcro for Festool husk sanding block”. There appear to be options, including two on Amazon, though those appeared to have a foam pad.
I don’t use the system, so I did not delve further.
No...I have the 3 foam versions and also the plastic HSK-A 80 x 130.
I think they don't offer the option because it's a hand sander and not subject to the heat and sanding forces that can be generated by a motorized sander.