Can I do 10 Gallon batches in my Half Barrel fermenter?

Yes

The thermowell sits pretty low in your beer anyway so you can pick up temps on your beer just fine doing 10 gals, the only caveat is our gallon markings don't go that deep into the fermenter so you'll have to roll old school (or roll like our competitors' products that don't have internal gallon markings!) for measuring your beer.

BYO magazine recently ran an article about doing gallon marking etchings (DIY) - you could always add a 10 gallon marking if you wanted.