http://www.espanol-ingles.com.mx/DictionaryEntryExplanation.png

Those dictionaries are English-English so I don't think how they format their entries is very relevant to us. A single language (i.e. kokugo) 
dictionary provides explanations of words while a cross-language dictionary provides glosses. If a cross-language dictionary has explanatory notes 
I would expect them to be formatted so that it's clear which part of the entry is a note and which is a gloss, even if it were in a more compact 
book format. 

I agree with you, though. Like I said, it's not an explanatory gloss, it's obviously a usage note, but tagging it [expl] at least lets users know 
it's not a regular gloss either, and it is the workaround we've been using the past couple of years. I'm not at all satisfied with this but I view 
it as a stopgap until we get around to allowing entries to accept just usage notes without other glosses (which I'm sure we'll get around to 
eventually). If it's just a stopgap anyway, I don't think it would serve much purpose to go through the hundreds or perhaps thousands of entries 
where [expl] is used like this and remove the tag, when we're going to have to redo it to put them into notes eventually.

We could perhaps continue this discussion on the mailing list in the "Glossless senses" thread. I really hope we can reach a consensus on how to 
proceed.

https://en.oxforddictionaries.com/definition/hey

I agree it's not a explanatory gloss, so let's not tag it as one.
Previously I would have argued it's necessary but I've changed my view.
All English dictionaries have definitions that start with "used" (see above for an example). They're not tagged in any way.
I think if we have a usage explanation as a gloss (because there's no fitting English translation), then there's no need to tag it as expl.

You're probably right. It really should be 
a note rather than an expl, though...

@@ -26,2 +26 @@
-<s_inf>at sentence end, to affect cuteness</s_inf>
-<gloss>teehee</gloss>
+<gloss g_type="expl">used at sentence end to affect cuteness</gloss>

Is that an appropriate translation for sense 3? "Tee-hee" represents giggling.
For words like this, I much prefer usage explanations over awkward translations that are more likely to confuse people than anything else.

@@ -26,2 +26,2 @@
-<s_inf>at sentence end</s_inf>
-<gloss g_type="expl">used to affect cuteness</gloss>
+<s_inf>at sentence end, to affect cuteness</s_inf>
+<gloss>teehee</gloss>

I think this is enough.

@@ -27 +27 @@
-<gloss g_type="expl">used to affect cuteness, oft. jokingly or in animated films/TV shows directed towards children</gloss>
+<gloss g_type="expl">used to affect cuteness</gloss>

https://www.urbandictionary.com/define.php?term=pyon
"the sound a rabbit makes is refered to as 'pyon' in 
japan. It's also used to make sentences sound cute."
ngrams
だぴょん	1539
だぴょーん	No matches
6+ hits paster hour on twitter for "だぴょん", 1 for "だぴ
ょーん", 1 for "だぴょ〜ん"
https://twitter.com/DD25_ijn_UZUKI/status/96350354201835
9296
"マルゴーマルマルぴょん♪　敵も油断しているから海面の乱反射に隠
れる潜望鏡をちゃんと探すぴょん。哨戒機もそろそろ注意だぴょん。
"

Really the entire [expl] of sense 3 should be in the usage 
note, but I can't come up with an English equivalent and I 
can't leave it empty, so. I did consider entering "pyon" 
but...

@@ -6,0 +7,3 @@
+<r_ele>
+<reb>ぴょーん</reb>
+</r_ele>
@@ -11 +14 @@
-<gloss>with a nimble jump</gloss>
+<gloss>(jumping) nimbly</gloss>
@@ -13,0 +17,12 @@
+<sense>
+<pos>&exp;</pos>
+<misc>&on-mim;</misc>
+<gloss>boing</gloss>
+</sense>
+<sense>
+<pos>&prt;</pos>
+<misc>&on-mim;</misc>
+<misc>&joc;</misc>
+<s_inf>at sentence end</s_inf>
+<gloss g_type="expl">used to affect cuteness, oft. jokingly or in animated films/TV shows directed towards children</gloss>
+</sense>

daijr, koj, etc.

@@ -9,0 +10 @@
+<misc>&on-mim;</misc>

daijs