Its still totally an option, just have to install a fan that exhausts the attic.... but then ur doing 2 fans instead of just the one.
Its super common for the bath fan to exhaust out the roof, or side of building, and you should view it as an upgrade to the existing residence.
Its not grow specific, when you are done growing, its just a normal bathroom with a fully functioning properly installed bath vent fan.
Dont go cheap. Get the higher airflow of an expensive bath fan, or get a nice inline fan, and just install a vent/cover in the ceiling.
For optimim use, ur gonna want to be pulling the air thru the filter, not pushing it, but if ur running an inline 4-5" fan, that shouldnt be too big a problem... (if ur venting out the roof @ 12-15' its oribably not much a problem either...)
Roof vents are STUPID easy to install, just scary to first timers, trust a man whos gone thru it. They are cheap, and install in a few mins. Mark the spot inside the attic (drive a nail thru) then cut the hole from the top side (its about not fucking up the shingles from the underside) lift the shingles in the area directly around the hole (measurements for this will be given with the vent paperwork) and slide in UNDER the layer/row above the hole. (sorry, make sure u get the collar in place in the hole first before that, its easier, trust me, lol) Ring of roofing sealant (or 2 rings even!) around the hole, put vent in place, nail in place with roofing nails, and more sealant over each nail head.
Attach vent tubing in attic to the collar, crack a beer, and pat urself on the back.
I put one in where we had a leak from the previous owner having some kind of small antenna (not a dish, holes were too small) and it was removed..... but they didnt seal the holes. Couple months of dripping b4 I located the problem...
This is my suggestion to you for so many reasons. It looks legit, and is an actual upgrade ot the home, not some gerry-rigged craziness....
Depending on the light ur using (if its HPS for example) you will over heat the attic in a 12hr space just venting there, not to mention the ever important rule of displacement... if ur pushing air in, air must come out to compensate... so wheres that (now stanky how humid) air coming out at?