I have this working in my lab and we will be looking to roll it out into production later this year for our mobile clients. I patched the gateway to R81.10 Jumbo 113 and enabled the SAML auth profile on the VPN Clients section.
Initially I got a white page only when attempting to connect. After collecting debugs from my phone it was related to a certificate issue. Worked with my cert people to get a new public certificate that included my lab gateways hostname and i was able to get redirected to azure AD on connection for sign in, and i connected to the VPN after successfully logging in.
Below is the debug from Capsule VPN Android that showed me my cert wasn't trusted and it was causing the issue:
* This can be ignored when using the Endpoint VPN solution (for lab) but it doesn't give an option to approve an untrusted cert on mobile.