I would like to push some static routes over an L2TP connection to a Windows machine.

Here is a method on Linux:
http://serverfault.com/questions/574121 ... during-con

It uses dnsmasq as a DHCP server, and when the windows client does a DHCPINFORM over the L2TP tunnel, it returns the DHCP Options with the static routes. I would like the ability to do the same on MikroTik. Is that possible?

 +1

This is essential to make L2TP/IPSec tunnels usable by non-technical users. We just need to be able to attach a DHCP server to a PPP interface somehow and I think that should do the trick. The rest of the capability of the DHCP options etc are already there.

Alternatively, if your client is Windows 10 you can use PowerShell command-lets to add routes to each VPN connection that are automatically toggled on and off with the VPN interface.
While it's not specifically what you are looking to do it is a viable solution to the problem of reliably adding or removing routes from client operating systems. Sadly, that part was never standardized with any of the VPN approaches. I don't know if that's because everyone wanted to have VPN client lock-in or if back in the day they thought the only viable reason to have a VPN would be to tunnel all traffic back to the VPN server.