https://reibuncnt.jp/3811#toc3
https://eigobu.jp/magazine/kagenagara#heading-39514

It shouldn't be hidden. I think iK is appropriate. 影ながら is probably a lot less common in edited writing. Over 50 hits for 陰ながら in Aozora but only one for 影ながら (with this meaning).

@@ -12,2 +12,2 @@
-<keb>陰乍ら</keb>
-<ke_inf>&sK;</ke_inf>
+<keb>影ながら</keb>
+<ke_inf>&iK;</ke_inf>
@@ -16 +16 @@
-<keb>影ながら</keb>
+<keb>陰乍ら</keb>
@@ -24 +24 @@
-<gloss>from behind the scenes(cheer, pray, etc. for someone)</gloss>
+<gloss>from behind the scenes (cheer, pray, etc. for someone)</gloss>

陰ながら応援	98285	68.5%
影ながら応援	45211	31.5%

see slightly longer note on 陰.  This *definitely* should be at least searchable as 影ながら.  Could mark [iK] I guess, doesn't seem that irregular really...("prescriptively incorrect kanji usage")  I leave it to you...

@@ -12,0 +13,4 @@
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>影ながら</keb>

陰ながら	169328	97.4%
蔭ながら	4249	2.4%
陰乍ら	313	0.2%
蔭乍ら	28	0.0%

I didn't realize ながら even *had* kanji. Interesting.

Not [uk]. 
陰ながら応援	98285	87.9%
かげながら応援	13557	12.1%

陰ながら	169328	87.0%
かげながら	25362	13.0%

The most common collocation is 陰ながら応援, which is well greater than 50% of all occurrences of this pattern.  It was so high I considered pitching it as an entry, but in the E->J direction it is obvious from the parts(still might be a reasonable entry due to its prevalence and some noteworthy nuances. 陰ながら応援しています could be glossed in its entirety as "I'll be praying for you", for example. ).

sankoku's definition is: 見えないところで、そっと。　

I've suggested a reordering of Robin's 2019 glosses with some tweaks.  

Google "陰ながら" "behind" and you'll get plenty of "behind-the-scenes" support. 陰ながら応援する in particular is apparently recommended for *business* contexts(otherwise it may seem cold), and can be directed at superiors.

Whole writeup on just that expression if interested;
https://news.mynavi.jp/article/20220912-2392937/

I think it's not a secret to the person you are supporting(for 応援 anyway). More like you won't actually be taking any action that would make it evident to the people around you that you are providing this "support".  All the strength of the English "I'll be praying for you/your success". (the article even provides this English "praying" equivalent).

In addition to 応援, 祈り, 支える, 支援, and other similar terms are used.  I imagine that most occurrences statistically will be of the "behind the scenes"/"from the sidelines" variety.

"I'm with you in spirit, but that's all I can do because I'm not participating in your endeavor in a way that I can provide any practical assistance". 

A weblio resource actually pitches "in one's heart":
https://ejje.weblio.jp/content/in+one's+heart
斎藤和英大辞典での「in one's heart」の意味
in one's heart
心中；内心；陰ながら；内々

"in my heart, I'll be supporting you/praying for you/....".  Maybe not the best gloss for this term, but I think the sense is spot-on. Well, I would say that if this weren't apparently most-appropriate for business use ;)

@@ -5 +5 @@
-<keb>陰乍ら</keb>
+<keb>陰ながら</keb>
@@ -8 +8,6 @@
-<keb>陰ながら</keb>
+<keb>蔭ながら</keb>
+<ke_inf>&rK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>陰乍ら</keb>
+<ke_inf>&sK;</ke_inf>
@@ -15,2 +20,4 @@
-<misc>&uk;</misc>
-<gloss>secretly (cheer, pray, etc. for someone)</gloss>
+<gloss>from behind the scenes(cheer, pray, etc. for someone)</gloss>
+<gloss>from the sidelines</gloss>
+<gloss>from a distance</gloss>
+<gloss>secretly</gloss>
@@ -18,2 +24,0 @@
-<gloss>from the sidelines</gloss>
-<gloss>at a distance</gloss>

gg5, daij

Indeed.

@@ -16,3 +16,4 @@
-<gloss>secretly</gloss>
-<gloss>behind someone's back</gloss>
-<gloss>unbeknownst to the parties involved</gloss>
+<gloss>secretly (cheer, pray, etc. for someone)</gloss>
+<gloss>in secret</gloss>
+<gloss>from the sidelines</gloss>
+<gloss>at a distance</gloss>