In reply to The Marmot:
Stamina is normally trained by traversing a few grades below your onsight limit, creating a gentle managable pump.
Best thing might be to build a small wall and traverse around on jugs replicating this. Additionally, this small wall would allow you to develop and maintain fitness.
If this isn't possible, I have not got a good idea. Maybe some exercises where you hang feet on, on a pull up bar for a long time, shaking out, creating a gentle manageable pump.
Generally for aerobic forearm training, you'd need to do it, around 3 times a week for a couple of months and then maintain.