Yes. I think that there's an application here that makes sense-
1.) What does your threat assessment tell you about the enemy's capabilities? Do they carry masks? Do they, if so, have spare filters?
2.) Do you face extensive networks of connected tunnels with rooms, storage areas, etc or are they very small, unsophisticated, and discrete from one another?
3.) Have your opponents shown a propensity or ability to reoccupy these positions after they've been cleared?
If they don't have masks, filters, nor de-con ability; if they exhibit some sophistication in their tunnel networks and; if they show a propensity or desire to reoccupy these positions after they've been cleared then I'd STRONGLY consider the use of a persistant non-lethal riot control agent like tear gas or any of the variants.
While not perfect and old tunnels would need a spot-check on the off chance that the enemy has acquired masks, filters, or (most important) a means to de-con, I'd check a few after the fact by about a week. Don't do so without being prepared for a fight. If you turn up empty and continue not to see masks among the opponent, there may be validity as an area denial weapon.
Needless to say, you'd know when you hit a tunnel that had been checked by your troops. So too civilians.
I'd personally use persistant nerve agent but you never know when some kid goes down a perfectly empty but lethal tunnel. Dead in minutes and not worth it. Most children would never get very far into a tear gas tunnel without getting the hell right back out.
Yeah. Under the right circumstance I could see it.