Here is the latest on this. 

https://gerrit.wikimedia.org/r/#/c/110095/ provides support for templated image options. It identifies individual image options that came from a template and provides VE sufficient information to make them editable. Parsoid can then serialize the edited attributes back to transclusions. See https://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#Transclusion-affected_attributes for how Parsoid conveys this information to clients (VE).

However, {{largethumb}} is still a special case. That gerrit patch also adds support for parsing largethumb as two image options. However, Parsoid cannot provide editing support for it, i.e. we cannot mark up both "thumb" and "261px" as coming from templates, that they are connected, and that VE cannot edit them individually. As you can see from the DOM spec (linked above), the spec is already fairly complex and we do not want to complicate the spec further to support this edge case. This is not just a complication for Parsoid, but for VE, and any other clients that need to deal with this spec.

That said, there are two options for how we can move forward.

1. Parsoid can mark images from {{largethumb}} uneditable so that VE doesn't attempt to deal with that template. This also means that the wikitext for these images will always be preserved and can only be edited in wikitext mode.

2. Parsoid makes the image editable. When the page is edited in VE, Parsoid preserves the image including {{largethumb}} transclusion when the image itself is not edited (even if other parts of the page are edited - which should be the vast majority of edits). But, if the image itself is edited, the {{largethumb}} template will effectively be subst-ed because Parsoid flattens out {{largethumb}} to regular options: "thumb" and "261px".

As of now, gerrit patch 110095 implements solution 1. as an initial step. However, in the long run, we feel solution 2 is a better solution that  provides editing support and also gradually moves towards reduced largethumb usage (by subst-ing largethumb images edited in VE). John Vandenburg has suggested in comment 3 (#c3 above) that if nlwiki set 261px as the default thumbnail size, editors could simply use |thumb instead of |{{largethumb}} and get the same effect. Not sure if that is viable, but collating all suggestions here in one place.

It would be good to get some input on these options.