Can exporters actually be customized?

[plantarum]

I'm trying to figure out some problems I'm having with exporters, and I found something I can't figure out. Exporters are defined as customization options, but it's not clear to me how you would actually customize them. For example, poly-r-bookdown-ess-exporter:

poly-r-bookdown-ess-exporter is a variable defined in ‘poly-R.el’.

Its value is #<pm-callback-exporter Rbookdown-ESS>
Original value was 
#<pm-callback-exporter Rbookdown-ESS>

  This variable has an alias: ‘pm-exporter/Rbookdown-ESS’.
  You can customize this variable.

Clicking on the 'customize' link, I see this:

I haven't actually changed this value, I started from emacs -Q with a minimal setup to load polymode only, and no customizations.

Clicking on the arrows, I see some documentation from the pm-exporter class, but no way to actually change any options:

In the source code, these 'customization' options are defined as the object returned by the function pm-callback-exporter. The customization type is defined as 'object, which I don't think is a valid customization type (at least according to the manual).

So I wonder if this is all a diversion, and these options might be better defined via setq, and users instructed to change them in their init file, rather than via the customize interface?

[vspinu]

These are eieio objects which are in principle customizable, but in practice it does not work very well. It's an upstream issue but I never go down to looking into it.

From elisp you would use oset or, if the slot is a list (like from and to) maybe object-add-to-list or object-remove-from-list.