In theory, what you are suggesting sounds like it would work, but I think the challenge is in explaining clearly WRT how it is accomplished through the DH control panel. For instance, where do you set the TTL on DreamHost? That seems like a whole 'nother can of worms (though maybe a smaller one...) It also matters A LOT whether your domain is ALREADY hosted at DH, or if you will be transfering it AFTER your set things up on DH (as is my case).
It sounds to me like wroxo DOES already have a domain and email setup on DH, and NOW wants to split the email off to a non-DH server.
The trick, as I discovered, is that if you want to use "mail" as your subdomain (as in "mail.mynewdomain.com") that conflicts with the default mail configuration on DreamHost, which is why you need to ENTIRELY deactivate DreamHost mail services for that domain BEFORE adding new mail A and MX records.
The further challenge, is that you CAN'T change the existing A and MX records set up by DreamHost by default... you can only add NEW ones. So I'm not sure if that throws a spanner in your suggestions above, since you'd only be able to change the zone records you'd created and not the existing ones from the DH default mail setup. Maybe this can be addressed through the "preference" value. I don't know.
Apparently, if you choose to go with a different sub-domain, something like "pop.mynewdomain.com", you can set that up WITHOUT turning off DreamHost mail services. Likewise you can also just change the MX record if the mail server is to have an entirely different domain such as "mail.differentdomain.com".
None of this is clearly explained in the existing tech documents, and a DH-Wiki would be fantastic to help others out in similar situations.
I'm a relative "mail server neophyte" so I hope I'm making sense.