I'd like to share experience with playing with mariadb.
From what I could feel, mariadb is a mere alias to mysql in the way it works for regular users. Of course, there are some tweaked things, which are probably required (and paid for development) by big guys. For common folks, I saw no major improvement and properly set Mysql 5.5 feels in my benchmark the same as Mariadb 10.1.

I played a bit with it and only learned that all my custom queries should be rechecked and tweaked again as plans are different with different indexes in explain. It doesn't make sense to do this work for me, because an already optimized query showed that the real major bottleneck is calculations in php scripts.

Whilst my query counts and groups a million rows in 0.5 sec (real website, not a benchmark), php script takes 1.5 sec to parse the results from table-view to ul/li-view with parents and childs. I mean, why should I care about database engine if I have this not-awesomeness?

I don't think it's a good idea to officially state Mariadb is supported. People who require mariadb or tokudb or whatever else besides mainstream mysql - not going to use couchcms. But things may change if, as you said, these branches come too far apart each other and there is a real demand.

In context of some major ecommerce website, there are so many other probable bottlenecks that a database engine is the last thing I learned to care about. Moving to PHP7 gives such a visible boost that no db can offer. PHP scripts and unoptimized queries - is the main pain. If you gonna be serious about hundreds (thousands) of shoppers on website and tens of requests per second, then (!) you'd create your custom tables to work with your custom queries. Couch will be only data-entry/data-management system but all the logic will be based on custom code derived from learning how couch sql queries work in <cms:pages /> and other fetching tags. So when this happened to me - I care more about scripts, because database is not a bottleneck in my real world use case with a lot of data.. I'm back on mysql 5.5 and not going to spend more time on trying mariadb or any other solutions, because this is a dead end.

Before I am spending a $100 monthly on hardware for a website, I won't touch other dbs. :) Average couch user probably still spends $1 a month and some of sites I built on couch live nicely on 0.1$ a month on an overshared hosting. :D My client's current expenses on VPS is about $20 a month for a decent config. Still a way to go ;)