because series in already used. Think of something like showing debug images based on 2 independent parameters for the model

ShallowCat10 so you mean like meta-data on top of the image? or another level of title series ?
because, the iteration field itself is an integer...

The iteration in my use case doesn't make much sense (I'm evaluation a model)

The only workaround I can think of is :
series = series + 'IoU>X'
It doesn't look that bad 🙂

I need to group images in a similiar way to what is done by iteration , but based on a string value which will be visible in the UI

Hmmm why don't you use "series" ?
(Notice that with iterations, there is a limit to the number of images stored per title/series , which is configurable in trains.conf, in order to avoid debug sample explosion)