A word or idiom for a person asked to use their expertise for banal tasks

When you do that, you are trying to avail yourself of someone's professional goodwill. (or take advantage of)

to avail oneself of something - "to help oneself by making use of something that is available."

• We availed ourselves of Tom's goodwill and let him repair the fence.
• The campers availed themselves of the first chance in a week to take a shower.

take advantage of - "to make use of for gain"

• I am glad to have your help. I hope I am not taking advantage of you.

There are several idioms to indicate that someone can carry out a task with little effort.

For example, with one hand tied behind your back

Used to indicate that one could do something without any difficulty: I could do her job with one hand tied behind my back

Similarly, with one's eyes shut

Without having to make much effort; easily: I could do it with my eyes shut

Oxford Dictionaries Online

Neither of these idioms is restricted to experts, but it is often used in the context of asking a favor of one who has skills beyond those of the requester.

I was thinking that "called out of retirement" could be used here. Of course, the phrase would have a humorous slant in this context.

I can't think of anything more specific for the moment, but if I do, I will edit this answer to match.

I describe the poor person's job as an "overqualified volunteer". Until the person learns, for instance, ten reasons not to fix computers for free and teaches relatives to Never Ask Friends or Family to Fix Your Computer. Risks are similar in other domains (medicine, car repair).