If you can get very accurate measurements to the distance saddle fitters, then I’m sure they can be quite useful. The validity of the whole process would depend on the accuracy of your measurements.
I am not sure how you could get a saddle reflocked at a distance, though I suppose it is done.
If you are doing this long distance, then you are going to be a very crucial link in the process, even more so than if the saddle fitter is visiting. I’d suggest browsing through the many saddle-fitting websites, and getting a sense of the different measuring systems and things to take into account. Saddle fitting guides on the websites of specific brands of saddle are not reliable. They tend to either mystify or minimize the elements of saddle-fitting, in large part because no one brand can fit every horse: each brand tends to have a specific tree shape it uses. But the saddle companies don’t want to be upfront about that, they would rather sell you a saddle and claim “we can make it fit anyone.”
Trees are like brands of sneakers. For instance, I don’t seem able to wear any Nike whatsoever; the arches are just too high, in every model. But I am fine in Reebok or New Balance.
I would absolutely budget in a total reflock as part of the purchase price of a saddle. You may not need it, you may only need a tweak. But most older saddles have rock hard flocking, and it could even be a negotiating point for a lower price.
This is assuming of course that you are buying a saddle with wool flocking. Foam cannot be reflocked. In general, high end British saddles use wool and high-end French saddles use foam. Also, in actual practice, “wool” is a catch-all term for “fibres, not foam.” I have seen some scary things. Inside an old German Passier saddle (from the 1970s): multi-coloured carpet fibre remnants. And one old County opened up to have weird horse-hair matted stuff inside it. Both were hard as rocks.
I should say that my 2001 Passier (bought secondhand basically unused as few years ago) is stuffed with nice clean white wool, so I don’t think the newer saddles have this problem. Ditto the 20 year old County I bought & had restuffed: the wool was packed flat and hard, but it was real wool. So I don’t want to slur the brands, they happen to be my current favorites (or at least the ones my horse prefers).
Saddles should get stuffed and restuffed with good pure new wool, carded but not spun. And a new saddles, or a restuffed saddle, will need tweaking as the stuffing settles and packs.