I understand your desire not to have to hump stuff around Bangkok, especially in the April heat.

I personally wouldn't leave any of my belongings unsecured in a lounge (rest room break excepted), even if staff are 'looking ater it'. BinSabai suggests (or I at least infer) it is acceptable to THAI staff to leave your bags in the lounge unattended/unsecured - I've never bothered trying as my carry-on is pretty much all the stuff I wouldn't want to lose. I'm a little surprised that lounge staff would allow this if you asked (but if this is the case so be it). If you didn't ask them, then I suspect that they probably wouldn't even notice.

You might consider alternatives (friends, hotels if you have status/contacts, left luggage locker).