I have never tried it, but I would put money on it that you could use a line level converter to get your headphone output, and then a pot to dial the volume down. This is assuming a simple stereo setup. If you have a subwoofer or a 5.1 system then it would take a lot more.
This isn't something that you will likely find already built, but it wouldn't be hard. If you have the room, you could easily use an in wall volume control for in wall home speakers. You can find these for as cheap as $20 on amazon and they usually can handle 100 watts, probably way more power than you are pushing. Then on the input side of the volume control you wire in a line level converter which is made for car stereo systems to take full power outputs and convert them down to RCA level (pre amp) levels. You can get an adjustable one for about $10 on amazon. From there you will have to find a male RCA to female 3.5mm stereo jack. If you search on amazon for exactly that you can find them for 1-5 dollars. Mounting this in a cab in an easily accessible spot would be the hardest part. The adjustable line level out would give you a way to dial the initial headphone volume in, and using a set of headphones with an inline volume control would be ideal if you don't want max volume all the time. There are tons of inline volume controls for headphones if you don't have headphones with them already built in.