Climbing hydrangeas are well suited to climbing trees :-) Those little rootlets from the climbing stems are very adapted to grasping and adhering to rough tree bark. But the plants can get big (as much as 60' tall) and heavy and do need some direct sunlight to bloom well. If the larch has been limbed up so that it has a clear trunk for a substantial distance, then it should do well and no harm to its support tree. IME, climbing hydrangeas simply stop climbing once they hit a lot of branches and foliage so never engulf a tree like you would see with something like English ivy. But they do take considerable time to establish - as much as 5-7 years before putting on much of a flower show.

If you search the Hydrangeas forum, you will find a lot of discussions about climbing hydrangeas that you might find helpful.