It won't be a road car. The engine regs align with the LMP1 prototype category. The McLaren P1 also doesn't fit in the GT category. McLaren could make the 12C into an LMGT car, but its aimed more at customer teams, and the 12C is already available as a GT3 spec car, and is doing well in that class across the globe.
As I've said previously I can't see McLaren taking on Le Mans alone, however if Honda want to join the party (and with Toyota and Nissan already in, its surely got to be something on the Honda heads desk) then McLaren are the logical partner to do it with. Honda's money and engine, McLarens brand and technical skills. Sounds like a great combination to me.