There are no lights of type mesh in Cycles, just meshes with emissive shaders that have an option for importance sampling. Those can be instanced, but for importance sampling memory usage will be poor, as if there was no instancing.
The easiest would be to tessellate them. If you want native support you’ll have to implement that as a new primitive. This is not something we’re likely to support in Cycles any time soon as we’re more aimed at animation / VFX use cases.
We do support rendering pointclouds as sphere, and hair as smooth curves. But that’s not the same as clipped spheres or cylinders.
Developers at NVIDIA are working on this, updating an earlier implementation from Tangent Animation.