NTC's New Japanese English Character Dictionary" (1993) usage 
notes.

Took out my gikun suggestion based upon what appeared to be 
popular opinion that idiosyncratic gikun should not appear 
appear in dictionaries. I do appreciate the discussion it 
stimulated though. Added usage notes and restricted use of 
3rd sense to 虚しい based upon NTC's New Japanese English 
Character Dictionary" (1993) usage notes.

@@ -11,4 +10,0 @@
-<k_ele>
-<keb>曠空い</keb>
-<ke_inf>&iK;</ke_inf>
-</k_ele>
@@ -21 +17 @@
-<s_inf>むなしい is gikun reading for 曠空い found in Meiji version of The Bible</s_inf>
+<s_inf>usually 空しい</s_inf>
@@ -27,0 +24 @@
+<s_inf>usually 空しい</s_inf>
@@ -33,0 +31 @@
+<stagk>虚しい</stagk>

and once again, i'll note that the few hits that are obtained all seem to be from the same source.

thousands?
"曠空く(むなし)" gives me exactly zero hits, as does "曠空く(むなしく)".
"曠空(むなし)" gives me 44 real google hits
"曠空(むな)" gives me 9

Rene, when you think about it, why would 曠空く get you 
any hits? Since it is uncommon gikun the reading should 
be given in parenthesis when it is used. You should 
always search for gikun this way, "曠空(むなし)" this way 
"曠空(むな)" or this way "曠空く(むなし)" (which will give 
you thousands of real hits). Unless it is well known 
gikun and only ever read with the gikun reading.

@@ -12,0 +13 @@
+<ke_inf>&iK;</ke_inf>
@@ -20 +21 @@
-<s_inf>むなしい is gikun reading for 曠空い found in Meiji period literature</s_inf>
+<s_inf>むなしい is gikun reading for 曠空い found in Meiji version of The Bible</s_inf>

for the record, 曠空く gets 60 "real" google hits (and at first glance, all of them seem to be related to this one bible translation) and 0 ngrams

Rene, you'll get a lot more hits on google if you try 
searching under the ku form 曠空く.

i am still not convinced that 曠空い is necessary or helpful. as i said, it gets one google hit (http://dqm.s198.xrea.com/patio-s-huri-/read.cgi?no=3379&l=2), which appears to be a recently written story.  aozora bunko and project gutenberg and the like have a large number of old texts available online, but 曠空い apparently does not appear in any of them.

if--as far as we can document--it is only found in a single meiji-era reference (=文語訳(明治訳)舊約聖書), i don't think it can rightly be said to be "found in Meiji period literature", and i don't think it needs to be an entry, as it appears to be just an idiosyncratic headword used by a single translator.

Good suggestion to split the senses. As for how 広 and 曠 
are related, in modern Japanese writing 広 is often 
substituted for 曠 since it shares the same onyomi, meaning 
and form (simplified form without 日.  昿 is the true 
simplified form, but 昿 is not a jouyou kanji so 広 is 
substituted instead）.　However, since the gikun reading むな
しい for 曠空 is what is found in the literature cited (文語訳
(明治訳)舊約聖書), modern equivalents maybe don't need to be 
included. Personally, I like seeing gikun readings in the 
literature as poetry for the eyes, so I propose at least 
including 曠空い with explanation "むなしい is gikun reading 
for 曠空い found in Meiji period literature".

@@ -10,0 +11,3 @@
+<k_ele>
+<keb>曠空い</keb>
+</k_ele>
@@ -16,0 +20 @@
+<s_inf>むなしい is gikun reading for 曠空い found in Meiji period literature</s_inf>

"曠空い"　gets one google hit; "昿空い" gets zero.  i think these variants are too obscure to be worth recording. also, i'm not sure what makes the 広空い version a modern equivalent, as 広 is not related to those characters as far as i can tell

splitting senses

@@ -11,9 +10,0 @@
-<k_ele>
-<keb>曠空</keb>
-</k_ele>
-<k_ele>
-<keb>昿空</keb>
-</k_ele>
-<k_ele>
-<keb>広空</keb>
-</k_ele>
@@ -26 +17,2 @@
-<s_inf>むなしい is gikun reading for 曠空い ; 昿空い ; 広空い</s_inf>
+<gloss>empty</gloss>
+<gloss>void</gloss>
@@ -27,0 +20,5 @@
+</sense>
+<sense>
+<pos>&adj-i;</pos>
+<gloss>vain</gloss>
+<gloss>fruitless</gloss>
@@ -29,3 +25,0 @@
-<gloss>vain</gloss>
-<gloss>void</gloss>
-<gloss>empty</gloss>
@@ -32,0 +27,4 @@
+</sense>
+<sense>
+<pos>&adj-i;</pos>
+<xref type="see" seq="1642360">虚しくなる</xref>

明治元訳 version of Bible uses gikun readings extensively 
throughout.

I'll be adding a bunch of these readings as I make my way 
through it. You might want to only include 曠空 because 
that is the only one of the three gikun usages I've run 
across so far. The other two (昿空 ; 広空) are modern 
equivalents, but I see no problem including them as well.

@@ -10,0 +11,9 @@
+<k_ele>
+<keb>曠空</keb>
+</k_ele>
+<k_ele>
+<keb>昿空</keb>
+</k_ele>
+<k_ele>
+<keb>広空</keb>
+</k_ele>
@@ -16,0 +26 @@
+<s_inf>むなしい is gikun reading for 曠空い ; 昿空い ; 広空い</s_inf>