......hook up a battery tender to the + post and the frame in engine compartment without any problems? Would be a lot easier than getting to the battery.
Don't think you are suppose to.. They come with a pig tail to leave on the battery for a quick connect.
I do! GPMINI has had a battery tender on it for more than a year, connected under the hood. Low currents so no worries doing it.
Remember that the alternator on the engine is connected at the front too - it's not a big deal to connect there. If the battery charger is high current, you need to be careful to get good connection so nothing gets too hot.
I connect a battery charger to my car at least once a year in the summer to top off the charge before winter comes. So you shouldn't have any issues.