I hadn't actually noticed that both kanji in 歎称 appear in the other forms.
This is an interesting case. It's a rare arrangement of non-rK kanji. I think we can stretch the definition of rK to include forms like this as it's still the use of kanji (as opposed to kana/okurigana) that makes the surface form rare.

I think given the low frequency of the compound itself the rK is appropriate.

I thought that since 歎 and 称 both appear in non-[rK] forms, 歎称 would not be considered [rK].
(This is probably trivial, but I want to make sure I'm understanding this correctly.)

gg5
嘆賞	1720	85.1%
嘆称	104	5.1%
歎賞	198	9.8%
歎称	0	0.0%

@@ -7,0 +8,3 @@
+<keb>歎賞</keb>
+</k_ele>
+<k_ele>
@@ -11,3 +13,0 @@
-<keb>歎賞</keb>
-</k_ele>
-<k_ele>
@@ -14,0 +15 @@
+<ke_inf>&rK;</ke_inf>
@@ -24,0 +26 @@
+<gloss>laudation</gloss>