OK. I'll align 事後 (for which 事後の is more common.)

PoS tags should be used to express the syntactic function of words.
The syntactic function of 事前 is that it can be used adjectivally with the particle の
and adverbially with the particle に.
The first fact is expressed by the tag [adj-no].
The second fact cannot be expressed with our current PoS system.
Usually such usage is implied by an [adj-na] tag, but this word is not [adj-na].
So we will have to add a note.

As I have explained at some length before, the tag [n-t] should either be abolished or reserved for
temporal nouns that can be used adverbially without attaching the particle に.
The [adv] tag should only be used for adverbs. An adverb is a word that can be used adverbially
(i.e. to modify a verb or adjective) as it is without attaching any particles.
I cannot find any evidence that 事前 is an adverb.

@@ -15,2 +14,0 @@
-<pos>&n-t;</pos>
-<pos>&adv;</pos>
@@ -18,0 +17 @@
+<s_inf>often as 事前に</s_inf>

Maybe this will work for now. 事後 needs to be aligned.

@@ -15 +15,2 @@
-<pos>&n;</pos>
+<pos>&n-t;</pos>
+<pos>&adv;</pos>

G n-grams:
 事前	12430961
 事前に	 7971624
 事前の	 1251393
 事前で	    5768	
 事前が	     571
 事前を	     485
 事前な	     216

Pos tag [n, adj-no] and mostly adv glosses
N-grams show mostly 事前に but hardly any 事前な.
How do we handle this?
Maybe a note, 'often as 事前に'?
(We could really use an [adv-ni] PoS tag for cases like this.)