Perhaps this callback on UI layer could be used.
https://developer.playcanvas.com/en/api/pc.Layer.html#onPreRender
Blend texture to framebuffer … this basically means to render full screen quad using the bloomed textured. As an example you can study post processing effects, such as bloom … they internally render multiple full screen passes for blurring the texture. So you’d need something similar.
And also I’ve relalized … if you only render few objects into the bloom texture and bloom that, it would not interact with the scene in cases something is rendered over the top. Example: you have a character you bloom, but in the scene its only half visible from behind a wall. This solution would bloom and make visible even hidden part of the character. To work around it, you’d need to do a depth pre-pass to your bloom texture - basically render the whole scene bit disable color rendering, and only write to the depth map. That would cause the character to be properly occluded.
Completely different way to approach it would be to use alpha channel of your render target to mask what needs to be bloomed. This way you would render the scene only one time, and during the opaque rendering, change the alpha output to mark meshes you want bloomed - for example character would output 1, everything else would output 0. Then at the end of opaque layers, you’d use this texture to apply bloom based on this alpha channel.
None of these are very easy to set up to be honest.