Orcus will likely be along to correct me
but here goes.
I was just pondering a humorous d20 project (Book O' Badgers) and a thought hit me. Suppose I take the Water Template ( closed content) and add it to the dire badger (OGC) to produce the Dire Water Badger. Can this be released as OGC?
My knee-jerk reaction is, "no." In fact, you can't even publish the Dire Water Badger, with template applied, without permission of the copyright owner on the Water Template. You're publishing someone else's closed content - a no-no without their permission.
Simple rule: If you didn't create it, start to finish, you can't turn it into OGC. (The exception is when you have written permission from the copyright holder, of course).
That said, if the Water Template was published under the OGL, even if the publisher says it's "closed" it may in fact be "open" because it is arguably game mechanics - which are *always* open per the terms of the OGL even if the publisher doesn't want them to be.
IOW, it's quite possible the answer to your question is, "the template is OGC by default, therefore your question is meaningless as both the base creature and the template are OGC so clearly the merge can be OGC." But you have to consult your IP lawyer on that one.
I would argue that the actual answer to your question is, "yes" (not the knee-jerk "no") because applying a template is following rules material - which if published under the OGL, makes it automatically OGC. OGC applied to OGC equals OGC.
IANAL, but that's my two bits.
--The Sigil