Is this feasible? If yes, this would be a big change for us, so any obstacles or warnings you could give us in advance would be much appreciated, especially about any potential impact on our SEO rankings.
A domain name (second level + first level domain, e.g.
maindomain.com) and a subdomain name, such as
whatever.maindomain.com only relate to each other in the DNS entry; other than that, they live totally independently from each other.
Meaning that you should have no problem whatsoever pointing
maindomain.com to any given IP number (e.g. to a WordPress server), and
whatever.maindomain.com to another, e.g. to a Galaxy instance.
SEO-wise is this setup not exactly optimal, however, as search engines treat them as two different animals, with each having their own separate ranking.
“Link juice” can passed to one another though by carefully selecting which page of one domain links to which other page(s) of the other. I.e. if you, for any reason, can only get valuable backlinks to certain pages of your main domain, you can pass the accumulated link juice to your app at your subdomain by creating links to where it seems appropriate. (This part is of course purely SEO, it has nothing to do either with hosting nor with Meteor.)