I've had them jam on several occasions. From memory there is a slot on the inside of the brake assembly, through which you may be able to get to the adjuster, but it's pretty hard to get in there. I've always been able to get them off (hot) without adjusting though.
First park on level ground and chock the wheel. Remove the circular clips that hold on the drive shaft joiner covers, then remove the covers, pins and the shaft. With a hammer and drift, tap the edges of the drum outwards - first from one side of the drum, then 180 degrees opposite. It may take a while, but with the right size hammer
you should be able to get it off.
Chances are the slave cylinders are corroded and seized, which happens when you don't use them. With the drum off, you should be able to back the adjustment off enough to get going again, or repair the slave.
Like you, I never used the brakes, but another operator used them occasionally. Only ever had it happen when he was driving, till we worked out what was happening.