Hello,
would it be possible to build a mash tun from a cooler without drain pipe by connecting a hose to the tubes at the bottom of the cooler and siphoning wort out?
Yes it is possible, I have seen pics of it done. It may be a bit of a hassle, but it should work if you provide a good drop for the siphon. Or you could just simply drill a hole in the cooler.
Could also be done w/ a piece of braid and a section of racking cane w/ a 90degree bend and some tubing. Hell, since it is preboil, you could even simply give a suck on the tubing to start your siphon.