Several options, do whichever works best for the situation.
You can disconnect the boot on the ducting and pump air down into the cavity. It won't have the volume that it will if you cut out or pull up a section.
Removing subfloor gives you much better access. Not hard to do, we've done it to dry and we've done it for mold work access. Just keep the crawl under negative air so you don't get contaminated air in the living space.
You can also pump the air in with layflat with the unit outside chained up and under cover. Not ideal but if you have no other option, it can be done.