Specifically in the Army National Guard, is it possible to attend training or resident courses on non-paid orders status?

This should answer your question; Per NGR 350-1 para 3-6
3-6. Training in non-pay status
a. Battalion and higher commanders are authorized, with the consent of the individual, to order IDT non-pay assemblies to conduct or participate in appropriate training as listed in Army training and evaluation programs and special schools such as noncommissioned officer and junior leadership schools.
b. TAGs may authorize issuance of written orders or other authorization governing an IDT non-pay duty status for individuals or units volunteering to participate in DSCA programs if participation is determined to contribute to the training experience of individuals or units.

I tried to do this route many times, and it was denied multiple times. I knew individuals at the school house and would have gotten a slot, and was willing to go out of pocket without a DTS.

Luckily, I somehow got funded for the school with extra AT money.

Although an option, I have never heard of this being approved. The only time I was on this type of order was when I was going in front of my flight board at the end of the fiscal year and there ended up being no money left to go on actual orders.

We put soldiers in an Inactive National Guard (ING), this allows those soldiers who are experiencing a hardship or just do not know if they are going to reenlist the opportunity to muster once or twice in that year, but do not have to attend IDT or Annual Training. Once their ING status is up, they either ETS, extend or reenlist.