Hi there!
Indeed as of 2.5 you will need to modify preseeds (cloud-unit) to configure source routes.
MAAS 2.6 will feature source routing (or as we will call it, support for multiple default gateways).
A few questions. When you configure such environment, are all your machines configured with source routing or only a select few ? Or the ones that connect to multiple subnets?
@mpontillo you may have more questions?
As far as work around, I suggest you take a look at Customizing MAAS deployments with cloud-init to customize your deployments!