I use the barrel storage solution (kmeta and citric acid) from that MTF link posted above. Although if I'm doing a barrel aged beer and want the barrel and spirit character I try and fill as quick as possible. Storage solutions will strip barrel character for sure. Just not sure how fast it strips the flavor. And burning sulfur sticks sounds like a pretty good time ....unless you like your eyebrows and limbs.
How long are you needing to store it before filling with beer? Also a quick note to NEVER sulfur a barrel that has previously held distilled spirits of any kind. Folks have done this before and it's not pretty.
I have a 5 gallon Balcones barrel. If mine is going to be empty in between beers, I just dump in 1.75 L of bourbon, purge with CO2, and leave it until I'm ready. I swish it around and rotate the barrel every time I get a chance. This is a good short term solution. You can easily go 2-4 weeks without issue. Plus it helps sanitize the barrel and soaks up more spirits. Its win win.
Burning sulfur is only a problem in barrels that had spirits. In that case it is dangerous, an explosion can happen as you alluded to.
Ah. Doing a big group brew or are you needing to brew 5-6 times to fill it? Filling it any less than 80% full is just asking for trouble. I would recommend as full as possible. I generally overflow a bit to make sure there's as little headspace as possible.
I am filling it with some friends. Maybe. It's a hard sell to get five guys to make double batches to put in a barrel that may net a sour beer accidentally.
