I'm no Air Force expert, just have several of them and have had a lot of discussions with Doug who IS an AF expert.
Yes, he makes a regulator but it is not generally for sale. Note that it is LARGE and requires a big bottle so portability becomes a problem.
As per him, the problem is the power from the Texan comes from the whole bottle being the plenum (with a fast inline flow) and if you regulate you lose that and the rifle becomes much weaker.
You CAN regulate the other AF platforms with smaller calibers but not the larger, air hungry Texan platform... especially not the .357 or .457
It (Texans) just can not be compared to conventional valved pneumatics
I don't have permission to show you pictures but one really would be 'worth a thousand words'.