I think this approach is sound. I certainly can't think of any better way of handling it.

GG5 daijr koji
30,200,000 わっ
 1,300,000 ワッ

* Add spelling わっ (more common)
* Add alt senses – the わっ = boo! is v. common and important (also “ugh” sense)

Note the entries:
わあ
わっ
わっと
GG5 puts わっ as a pronunciation of わあ, while 研究社中 and the 国語辞書 (daijr, koj, shinkai) have them separate.
Everyone has わっと separate, as that’s an adverb (quoted sound).
Following our standards (w/o kanji, a bit tricky to do 2/3 analysis), I think best is to:
* include わっ as a reading on わあ、わあっ、わっ, as it’s used for all the senses of わあ
* have a separate entry for わっ in the senses that don’t overlap
The only sense that’s similar is that わっ is used for both startl*ing* and being startl*ed* (you don’t use わあ to startle though, AFAIK: dicts don’t have it, and you really need the っ to startle) – I think this is clearest as a cross-reference.

@@ -4,0 +4,3 @@
+<r_ele>
+<reb>わっ</reb>
+</r_ele>
@@ -9,1 +12,11 @@
-<gloss>ulp</gloss>
+<xref type="see" seq="2087950">わあ・1</xref>
+<xref type="see" seq="2087950">わあ・1</xref>
+<gloss>boo! (sound used to scare someone)</gloss>
+</sense>
+<sense>
+<pos>&int;</pos>
+<gloss>ugh (dissatisfaction, aggravation)</gloss>
+</sense>
+<sense>
+<pos>&int;</pos>
+<gloss>ulp (consternation)</gloss>