The Check Engine light is used in the Capri, at least in the US here - it's used to warn of a fault & also used in the diagnostic to pull the codes.
There are two lights un-used on the XR2 - "Power Mode" and "Manual Shift" MS light was used on the autos, and I think PM was intended for the autos as well, but not used? I don't own & really haven't driven the auto capri much so maybe someone who does can enlighten us.
On my red XR2 I've re-purposed the power mode light - I wired a simple circuit to the stock boost cut switch ( I have a chipped ECU, I can safely run 16psi) which activates at ~11psi to turn on the "power mode" light. It's a simple little mod I like quite a lot. This required adding a new wire to the gauge cluster's harness, as there was no wire in the harness originally. You can follow the traces & corresponding wiring on the back of the cluster. Here's a crappy video of it operating:
http://teamcapri.com/forum/index.php/topic,2875.0.htmlBut let me get this right - you'd like to use both the stock gauge & an idiot light, together? By the time the light comes on you're likely in a heap of trouble anyways. Fortunately for the Capri sudden oil pressure loss is not a common issue, or even really an issue at all. The hydraulic lifters clacking away are an audible warning of low pressure, as well.
Why not install a proper aftermarket electronic gauge separately in the dash? or even add it to the stock cluster inside?
TLDR; yes it's actually quite simple to re-purpose one of the unused warning lights. Just add your own wire to the appropriate part of the cluster
Sidenote: your XR2, is a turbo? With a tickford badge, yes? It's genuine?