I would say you are opening yourself up to a whole world of pain.
There is an established, security reviewed, gold standard pattern for applications to communicate via a browser to ADFS using WIF.
So what you are essentially trying to do is re-invent the wheel by writing a web service that implements all the browser functionality e.g cookies. Your solution loses all the advantages that ADFS provides e.g. SSO, Federation, HRD etc.
And now you want to dig the hole deeper by writing your own STS.
The browser pattern is exactly what Microsoft use for Windows Live, Office 365 and Azure Active Directory.
So if it's good enough for Microsoft, why does your architect not want it?