I need to provide remote access to several internal networks (all without Internet connection). Networks should be separated one from another.
Currently I'm using independent OpenVPN servers, hosted on Windows Server machines located in those networks. Internet access is provided by 3G modems with static external IP.
I need to connect a new network, this one does not have a server where I can run OpenVPN server. While looking for some hardware to run it I decided to give Mikrotik a try, it looks decent and the price is excellent.
Also, as an option, I'd like to get rid of the static external IP option - it currently costs way more than the data transfer itself!
My idea is to reverse the client-server direction:
Remote network with a RB411U (with a 3G dynamic IP link) is a VPN Client , and connects to a VPN server hosted on my office server.
When access to remote network is needed, a VPN connection is made to the same office server, which relays traffic to remote network.
Does this idea make any sense in a real world application?
The on-site hardware will be a few hours' ride from me, and unattended, so I want it to work 24/7. In case if anything goes wrong, it should try over and over again to recover access.
The VPN tunnel must be in Layer2 as non-IP protocols are in use.
Which VPN option would be the best suited for the task? I thought about using OpenVPN as I'm familiar with it, but I've read it's not fully supported in Mikrotik?

Yes, OpenVPN in RouterOS is lacking a bit. Mainly because it doesn't support UDP and LZO. Plus some other features, but these two tend to dissapoint most people. But there is L2 TAP mode, so it could be usable for you.

You can always download Cloud Hosted Router (CHR), which is virtual machine with RouterOS, and test yourself, if it can do everything you need, before you buy any MikroTik hardware.