Wow--have the laser level/light that I use for squaring up large quilts, but had never thought to use it to throw a cut line on the batting--GREAT idea. Ditto using the magnetic strips to hold tension on quilt you are floating--does it hold it firmly enough? I usually pin--and have the t-shirts with holes to prove it!
Besides the items mentioned, I have a little air compressor for blowing out dush along the long arm track; plus you can get canned air cheaper there, and I have a folding push cart that I use all the time to lug my all the stuff that goes in and out of guild workdays