Connecting P2S <-> S2S <-> On-prem is indeed possible without BGP. You simply need to include the P2S client subnet in the "private subnets" configuration when setting up the VPN in Meraki.
Use the following Guide: https://documentation.meraki.com/MX/Site-to-site_VPN/Configuring_Site_to_Site_VPN_tunnels_to_Azure_VPN_Gateway
Sample values:
Public IP (Virtual network gateway Public IP): 53.158.17.57
Preshared secret: SomeThingVerySecret
Private subnets (Virtual Network Address Space, P2S Address Pool): 172.16.0.0/16, 172.31.255.0/24
Note: Including the Point-to-Site Address Pool is the key!