"View only" user fields in Portal

As a product owner with external stakeholders, we all love our ideas portal as a communication tool; the stakeholders can submit and see that we are working their ideas/bugs/etc without falling in an email pit and getting lost forever. 

We have added a custom field to also give the client visibility into t-shirt sizing once their idea has been vetted.  The problem is, when they submit the idea initially, the t-shirt size is available for THEM to enter, and that's not cool (because then everything would be small). 

So, it would be super useful to be able to set custom fields as "view only" on the portal, so fields are editable only in AHA, but visible in the portal.