Are you using MaaS for bare-metal server or VM deployments? And you need some mac filtering that you want MaaS to ignore dhcp discovery packages which comes from particular MAC address(es) ? If so, you can simply add similar snippet of isc-dhcp configuration to your MaaS. (Settings > Dhcp Snippets) set the scope as Global.
After adding, enabling and saving it. MaaS will not send any PXEboot reply to system that has the MAC addresses above.
Happy Deployment 🙂