We've deployed a number of Web Application Proxies for various clients and they all have the same problem in that the WAP is incompatible with Safari. We've tested with Safari 7 (10.9) and Safari 8 (10.10). Safari can load the AD FS Forms Based Authentication page internally or over VPN when it isn't going through the WAP, but it cannot load it through the WAP. On these same Macs you can access through the WAP with Chrome or Firefox without any issues.
Here is another thread that talks about what seems like related/similar/same problem.
Thank you.