Sporadic hangboard session for maintenance?

I'm an intermediate boulderer/climber who climbs outside about once a week and then on a moonboard or homewall about twice a week. There are sometimes days or quick family vacations where I would like to fit in a session or two for maintenance and all I have access to is a hangboard. I don't hangboard regularlly. So, are sporadic hangboard sessions beneficial in a case like this?

I'm envisioning some sort of repeater protocol for these sessions, but welcome other opinions.

I think so. It kind of depends on what else you do training, if you literally are only active 3x a week then I'd say adding something like a lifting day (to balance muscles) or hill sprints (increase vo2 max) would be more beneficial.

What is your goal? Sport climb harder? Endurance? Power? Boulder harder? Boulder a certain grade? How much do you weigh? How tall are you? How hard do you climb? How old are you?

I like to do max hangs if I only have a short bit of time and want to stress my shoulder complex and fingers. Or Density hangs (failure 20-30seconds on smallest edge that you can muster that time) three sets. Shoulders engaged elbows engaged 160-140°.

Repeaters will help with your endurance but if you are bouldering 2x a week they may be hard to recover from. I like to do some very short raw strength/power routine for fingers then lift free weights. Swings and TGU with kettle bells are nice as well as shoulder/military press.

Such a difficult question to answer, it really matters what you want to work on and you should evolve and change your tactics for your personal goals. 

How often are the vacations/sporadic days? Do they just pop up or do you know in advance and can plan for a month or two at a time? If you are consistently getting outside/moonboarding 3 times per week, not hangboarding and that works for you and you are continuing to improve I don't think I'd be worried about trying to fit in a HB session if they are very sporadic. I'd probably opt for the rest day/week instead when you can't get out or go on vacation. I think we often get over concerned with losing our strength/power if we aren't doing something. If you don't work out on a vacation for 7-10 days you will be just fine and actually probably better off when you return. If you have a good handle on your schedule and you think you could get in a HB even once every 10-12 days without taking away your rest days you could definitely do something that would be helpful and effective with that. What you did would depend on your goals as stated in the post above.

It'll probably only help you maintain, unless you're doing 2x week then you won't be able to progress it much...

I would take a look at Steve Bechtel's 369 protocol for finger strength maintenance. It's easy to remember and forgiving on rest length so you can essentially do it anywhere you have an edge. And you can tweak it to your grip weaknesses, tiredness on the day and to some extent strength/endurance needs (though it is basically a strength workout). For supplemental hangboarding I think it's important you have a simple protocol that is easy enough to execute so you actually do it and log some meaningful load time on your fingers.  It's easy to get demotivated by more complicated routines. Time under tension is a very important factor in getting and keeping stronger fingers. That means having a maintenance protocol you will actually do.  SteveBechtel's 369 protocol