The primary function of the gluteus minimus is hip abduction, similar to the gluteus medius. The gluteus minimus is deep to the gluteus medius. The different muscles provide different attachment points in the hip, which provides them with more leverage during certain angles. Through a full range of motion, all hip abductors are used.
Since in a concentric move the gluteus minimus performs hip abduction (swinging your leg away from your body, hinging at the hip), the eccentric activity would be in resisting hip adduction.
You could accomplish this with bands. Put a band around your ankle, with the banded-ankle winged out. Slowly pull your leg back into anatomical neutral. Your gluteus minimus will lengthen under tension, counteracting the abduction.
The more typical way of doing this is the lateral band walk. You'll be activating the gluteus minimus and gluteus medius concentrically when you abduct (move your leg away) and eccentrically when you adduct (bring the leg back in).
Just looking at this part of your question:
I don't want any concentric load to the gluteus minimus/medius.
Then you wouldn't do the lateral band walks and would just focus on adduction, which would put all of the hip abductors into eccentric mode. I'll skip past the part about asking if this is a good or bad idea, but this is how you'd do it.