2 sheds.
What Tiggur says is not totally contrary to MATS pt1 as you say. Have a look at FIS MATS Pt1 5.1 (b), which I think you're thinking of is.....
'The controller may attempt to identify the flight for monitoring and co-ordination purposes only. Such identification does not imply that a radar service is being provided or that the controller will continuously monitor the flight. Pilots must be left in no doubt that they are not recieving a radar service'.
But it also says.......
5.2. ' In addition to the above, controllers will, subject to workload, provide pilots with information concerning collision hazards to aircraft operating in Class C,D,E,F, and G airspace when self evident information from ANY source indicates that a risk of collision may exist'......
I'd say that providing you make it clear that you're providing a FIS by establishing the 'verbal contract' and ensure radar derived information is passed with the 'believed to be' included you're actually covered. Now, as to doing that without an Approach Radar rating, don't think I want to go there!
Spiney.