I don’t see anything mentioning this specifically in Appendix H??
Yeah, that’s pretty crazy @mdp9! In this case it would be better to put studs in beforehand and maybe do dressage in the jump saddle?
I did an HT last weekend (Old Tavern, a brand-new HT at Great Meadow), and there was only an hour between dressage and jumping - with a goodly distance between the two. I had latish times, so was parked allllll the way down the “row” - and there would not have been time to go back to the trailer. Dressage was on grass so studs were already in, and my poor husband (who is 6’4" and 280#) schlepped my jump saddle, jump bridle, boots, other helmet, pinny, armband, watch, and jumping bat to dressage (luckily I had a friend who was kind enough to video dressage), and we changed out everything under a tree. Luckily my mare is very good and quiet which made this a piece of cake 
, and I got to the jump ring in plenty of time.
One year the Area Training Championships were at Waredaca; it had “rained like a MoFo” (as is typical there) a couple of days before the HT, and they were running the T 3-Day - so the footing was so trashed that they decided to move the Championships to the following weekend.
We had to get there early enough to walk the course(s), put in studs, tack up for dressage, ride our test, do a quick tack and clothing change, and be ready for SJ (then straight to x-country) about 40 minutes after our dressage ride time. Luckily since the only competitors were the TCH folks there were maybe 25 rigs parked - so less of a hike to the rings. I had a couple of additional sets of hands (students) who were able to help me get everything done efficiently and the whole HT was over from start to finish in less than an hour! No victory gallop, but we were all just grateful that they were able to get all the officials and volunteers back just for us 
Loch Moy has Twilight Eventing in the summer where you basically run all phases one after the other. I’ve done 3 of these (one with no help), and it required some creativity: studs in beforehand, dressage in jump saddle, use same bridle for jumping (they allowed boots in the dressage test IIRC), wear body protector down to the dressage ring (and carry jumping bat and running martingale), drop them under a tree, then beg someone to hold horse while running up to the secretary for pinny - once through with dressage, grab all jumping stuff, put vest on and head to SJ warmup. They are INCREDIBLY helpful and accommodating there, and put these on so that people can “school” a HT - not timed, no ribbons, etc. I was able to beg a water from the SJ steward while waiting for my turn, and they had a water cooler with paper cups; riders were downing water “on the go” and tossing the cups (picked up by volunteers.)
NOT my preference in terms of scheduling (especially at my age since my back and legs need a bit of a break, even 30 minutes is good), but the whole HT sure is over quickly!
I’m wondering whether @Janet can weigh in here?
