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I've been absorbing Japanese through osmosis for years, but I just started
learning it formally. The app I'm using presented these two sentences with the
same translation, I think just to show that the interrogatives like どこ can't
take は as their particle:

> 病院はどこですか Where is the hospital?  
>  どこが病院ですか Where is the hospital?

My question is, when would you use the latter? (My understanding is that the
first is more normal.) What's the difference in emphasis?

I think I get the difference for a different interrogator. If I understand
correctly, 先生はだれですか is neutral/putting emphasis on teacher, so it could mean
"Who (among us) is a teacher?" Whereas だれが先生ですか puts emphasis on "who," as in
"Just who _is_ that teacher." (I'm overemphasizing the distinction.) But I
don't see how that applies to どこ.
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usage
Difference between Xはどこ and どこがX
I think your interpretation of the distinction between 先生はだれですか and だれが先生ですか
is correct and it applies to 病院はどこですか and どこが病院ですか, so you can emphasize "どこ".

Probably it's a little bit strange since a hospital is normally big, so you
can find it easily and you don't have to emphasize "どこ" normally. Perhaps you
know the address of the hospital, but the hospital is mingled with other big
buildings. So you want to emphasize "どこ" in the case.

"だれが先生ですか" might be used when "先生" is mingled with a clerk in a teacher's
room. So, you need to emphasize "だれ" to discern the teacher from them.
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> A) 「病院{びょういん} **は** どこですか。」
>
> B) 「どこ **が** 病院ですか。」

The two sentences above surely have different meanings and usages.

As you stated, **A)** is the more "normal" or "versatile" sentence. It is used
to ask for the location of a/the hospital in a situation where the speaker has
little to no idea where a/the hospital might be in a town or area where the
speaker is a stranger. In fact, there might not even be a hospital there.

So, the replies to A) can be " _ **Turn left at the next traffic light and go
down a couple of blocks...**_." or " _ **I'm afraid there is no hospital
around here**_."

**B)** is used very differently in that both the speaker and listener know
that there is a hospital (nearby). Suppose you are looking at a campus map of
a medical school that you already know has a hospital attached to it. You want
to know exactly where on the campus it is located and the other person might
point a finger at the south-east corner of the map and reply 「ここ **が** 病院です。」
or just 「ここです。」.

A がｰanswer to a がｰquestion.

It could also be a small town or area to ask question B) in a situation where
you have a prior knowledge that there is a hospital there. Either you may have
had that knowledge for a long time or you have just learned it a moment ago.

Finally, A) can often replace B), but B) could not replace A) nearly as often.
That , of course, is because B) is more specific and A), more general. By
"specific", I mean to say that more specific situations/contexts are required
to naturally utter B) (as my example of a medical school map above).
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