Dedication to the God Anvallos
It has occurred to me that Metambala - if we assume this place-name is properly *[Ne]metambala - could be etymologized in a new and different way.
First, we must recall that the Lydney Park Nodens temple, often cited as a candidate for *Nemetambala, was in Ercing, a kingdom bordering on the Hwicce tribe (supplanters of the Dobunni). According to Professor Della Hooke (personal correspondence),
"The whole of that area [Lydney Park] lay outside the kingdom of the Hwicce (as suggested by the diocesan boundary) and must originally have been within Welsh district of Ergyng, a district around the Roman centre of Ariconium (?Cantref Coch. This is discussed by Bruce Coplestone-Crow in his BAR book (British Archaeological Reports, British series 214, 1989) Herefordshire Place-Names."
Readers of my blog will recall my discussion of the Anblaud made an ancestor of Arthur and other notable figures in Wales. This personage is associated with Ercing. The name means something like 'the very terrible [one]', from an intensifying prefix plus Clt. *blad-. What I'm wondering is if this Anblaud could be a relic of or substitution for an earlier Celtic god name known from Autun on the Continent.
-ambala, if we allow for some of the usual form and copying errors encountered in the RAVENNA COSMOGRAPHY (see Rivet and Smith's The Place-Names of Roman Britain), could ultimately derive from Anval(l)os:
We would then be looking at a Nemeto Anvalo or similar. Granted, this doesn't make much sense in the context of the Lydney temple, as we know the god worshiped there was named Nodens. But it could be that Anvalos was being used as a cult title or epithet for Nodens. Alternately, Metambala may be another site and not a designation for Lydney Park.
I emphasize that this notion is highly speculative and just as likely to be wrong as the other proposed etymologies for Metambala. I mention it only because, to date, it's really all I can come up with for this place-name.