You can certainly get someone else to do the welding for you, and they would probably be happy to work from your jigs as long as they were logical and didn't get in the way of the torch.
Welding is the universally prefered method of joining tubes and the prefered method of joining tubes in the racing world, except for bicycles. I assume it is also the prefered method by which the base frames are manufactured? So from a liability perspective it would appear the obvious choice. If weight isn't an issue then I can't see any reason not to weld frames together.
Temperature is often bounced around as a reason to prefer brazing or lugs over welding, but the critical temperatures for steel are in two bands (over simplification warning). There would be an advantage to a joining technology that was cold, possibly as low as 400 degrees or less, like epoxy gluing (would require very extensive lugs). Certainly by the time one is at 1000 degrees one is well beyond the range at which the temper of a heat treated tube would tend to be losts. The next bump in the road is the temperature at which steel burns. Gas processes shield the joints agains burning up so it doesn't mater that the tubes are heated to the melting point. The other thing about heat is the distribution of it. Arc welding done properly limits the heat affected zone. Brazing keeps pumping it into the joint so you may have a wider affected area, which is a negative, though the fact welding and brazing both work fine is evidence that these issues need to be identified but they aren't deal breakers.