how about using a parameter?

We workaround the lack of MTU in subnet like this at this time. But we have some hosts with multiple interfaces with different MTUs (eg: admin iface with default MTU=1500, Storage ifces with MTU=9000,and sometimes ifaces with MTU=1496) and these cases not easy to handle.
If we add the MTU directly in the subnet, we can provision hosts, linked to multiple subnets and all interfaces of theses hosts will take the right MTU for all ifaces.

• Adding optional 'mtu' field to subnets
• Modify provisioning templates to add MTU support and be backward compatible with foreman versions that don't support mtu in subnets (try if subnet respond to 'mtu'. If it's the case and the value is not blank, set the provided MTU, else don't touch anything).

Any pointers/advises are welcome.

I think you can achieve the same with subnet parameters today but I agree that built-in support would be nice. Do you plan to work on the patch? Also the template does not have to be backwards compatible I think, we update all shipped template with every seed and we lock them. Since templates are now version (branch per release in community-repo) we can simply rely on the feature being present.

Patches for foreman and templates are ready, I just wanted some feedback on this before pushing.

Obviously I can't speak for others but you have my thumb :-)