In Open GL, is it possible to render a polygon with a regular depth test enabled, but when the depth buffer value is actually written to the depth buffer, I want to write a custom value?
(The reason is I'm rendering a particle system, which should be depth-tested against the geometry in the scene, but I want to write a very large depth value where the particle system is located, thus utilizing the depth-blur post-processing step to further blur the particle system)
Update To further refine the question, is it possible without rendering in multiple passes?
You don't.
OpenGL does not permit you to lie. The depth test tests the value in the depth buffer against the incoming value to be written in the depth buffer. Once that test passes, the tested depth value will be written to the depth buffer. Period.
You cannot (ab)use the depth test to do something other than testing depth values.
ARB_conservative_depth/GL 4.2 does allow you a very limited form of this. Basically, you promise the implementation that you will only change the depth in a way that makes it smaller or larger than the original value. But even then, it would only work in specific cases, and then only so long as you stay within the limits you specified.
Enforcing early fragment tests will similarly not allow you to do this. The specification explicitly states that the depth will be written before your shader executes. So anything you write to gl_FragDepth will be ignored.