Is this something that can be done? I had a 2nd cable box intalled in an outbuilding. Using MOCA would save me having to come up with a WIFI repeater/extender solution or somehow routing a long run of Cat-5 through the back yard.
Alternative solution: Can I buy a 2nd modem and stick it onto the coax that's feeing the cable box in the out-building?
No a second modem requires a second account... Use two Moca adapters . Do you need cable service out there for tv? If not you can use a piece of dedicated RG6 coax.... I'd properly install grounding blocks at each end to reduce spike and lightning damage or better yet, use a decent wireless acces point with antenna pointed towards the building.
It must be 2.4 gHz as it's illegal to use 5.8 outside at hi power.
How far is it away?
My wrt54g running DDwrt get's near 300' at 70 mW power, I can get 500' at 150mW, wouldn't run it up to 254 as that will overheat the transmitter. A directional antenna helps a lot more than raising the power.
Also, you'll get better range by lowering the speed and bandwidth
If you still need to use the outbuilding cable box, it may be tricky.
Where is the splitter that feeds the outbuilding? It will need the moca box preferably after it with a POE filter isolating moca from that splitter for an ideal setup.
If not, you'll need to put the POE filter before the main splitter and attempt to use in house coax to get to wherever you put the moca box .
If there are more than 4 devices and or an amplifier it get's really tricky as most amps are not capable of passing moca.
They also can destroy the s/n on the modem and cable boxes.