For yeras I've stretched tubular basetape by inflating the tires to about 130 psi not mounted on a rim and letting them sit for a couple hours this way. They turn inside out and can be gently rocked over the centerline. It makes an easy way to apply the base coat of glue and very evenly stretches the basetape with little chance of it peeling loose or tearing it from pulling it. This is also an effective way to reglue basetape after applying latex. I have a carpet cleaning business these days, having left much of the bike shop to gather dust... you can find pure latex from carpet supply stores. It's used for "buttering seams after cutting the carpet to dimension before joining the seams with seam tape. It's the same as Jevelot's tire life. A quart will last a lifetime, also.
Paul Brown
Cycle Dynamics
Santa Rosa, CA