Have you tried making a Network Device Profile for this vendor product? You can start by creating a RADIUS dictionary definition into ISE to populate the custom attribute(s) you need, and then you can craft your own MAB and 802.1X authentication detection Rules based on that. It means that ISE will do all the attribute matching/checks for you, based on your custom logic.
If you're only after one RADIUS attribute "DHCP Option" then you could also create that one manually, and ensure you set it as a STRING and has tick box set for "Allow multiple instances of this attribute in a profile". I just made up the Attribute ID "5" (you can get the true values from a tcpdump/wireshark decode)
Once you have this Device Profile, you can apply it to your 3rd party switch Network Device configuration (instead of the default Cisco value). But also be aware, that any Authorization Profiles sent to such a custom device, must also be tagged with this Vendor Profile, or they must be "blank" (i.e. apply to all vendors - as an example of this, the ISE built-in Access-Accept is vendor neutral)
I believe that, by adding the 3rd party device into ISE's RADIUS Dictionary, you will have access to these attributes in your Policy Set logic, and also Profiling logic (Type:RADIUS Attribute Name: VendorSpecific Operator: EQUALS Attribute Value: {VendorID})
I reckon this should be worth a try.
