PT from root side of the weld (inside the pipe) could be achiveable and will give you an idea of any surface disct. up to 3mm, however, it would be the first time I've seen this practice used.
PT on the root, before hotpass is not a good idea. You have a much greater chance of inducing hydrogen...
You main concern is cold cracking in the root due to the butted pipe acting as a heatsink. PT on a root before the hot pass is not going to work becase it is too hot, the dye will simply burn-off (can't be done on a hot surface anyway as per ASME V).
Dry MT is the most common NDE technique...
