Matt, I like your thinking on the 3 short (maybe 1/2" tall) rubber feet.  
How long do you think they would have to be before someone even notices?  I agree, it should pass muster.   I did have someone once call me on having 4, 1/4" thick x 1+" dia. rubber pads (stick on).  I took them off for a bigger match.  Did not want to chance it.  Or bring a large mallet to the line and beat the heck out or your shooting position.  Then again you may run afoul of some "improving your shooting position" ruling.   I have been using a HD moving pad the past 2 years.  Works great on grass.  It does not slip around like a typical Midway nylon mat.  No good in the rain

, so I bring both.  Like Danny Biggs, I cut an opening in the 
nylon mat for the bag to set on dirt, hopefully grass.