There is no support for filtering on parameters. To filter it needs to be able to query on that information for it to be efficient database wise. What is this date supposed to be doing exactly? Comments already log their date. It maybe better to just add that information to the Asset, which you can filter on with the built in filtering functionality. Example as follows for a custom asset in a comment field.

profile.[user_id].field.[field_id].[cb_mydate],profile.[user_id].field.[field_id].%,profile.[user_id].field.[field_id]

The first asset on a stream is always used for storage. So it'd always store with the value of the field cb_mydate added to it. The following assets would be sure it shows all comments regardless of the cb_mydate value and then also without cb_mydate entirely. You'd then just add a filter to the comment stream on the following asset.

profile.[user_id].field.[field_id].[cb_mydate]

If cb_mydate may not exist for the user you'll want to add an IF condition like the following to only use this behavior if it exists.

profile.[user_id].field.[field_id][cb:if cb_mydate!=""].[cb_mydate],profile.[user_id].field.[field_id].%,profile.[user_id].field.[field_id][/cb:if]

You won't need any special custom code for this solution and should just work. You don't have to use a comment field specific asset either. For example the below is entirely with a custom asset that's also user specific.

Asset:
comments.[user_id][cb_mydate],comments.[user_id].%,comments.[user_id]

Asset with IF:
comments.[user_id][cb:if cb_mydate!=""].[cb_mydate],comments.[user_id].%,comments.[user_id][/cb:if]

Filter Asset:
comments.[user_id].[cb_mydate]

Hmm, I start to understand better what assets are good for. Still a bit difficult. Is there any documentation about this in addition to the quick help on the modules asset field?

The parameter includes several examples in addition to my explanation and examples above.

An simple explanation of my need is that registered users can comment on multiple but separated topics. An user can switch between topics through an update to his profile that holds the id of the selected topic in a custom field.

Any user can read all posted comments on a certain topic and just this one (when he has switched to it), regardless whether he has posted any on its own.

Then you should just need the following asset usage.

topic.[cb_topic]

The cb_topic field would hold the topic id.

I like to have a single activity module that displays all comments posted on a certain topic once the has switched to it.

That's doable with a module using the following asset.

topic.TOPIC_ID_HERE

Would that mean that have to set up a custom asset like 'field.[field_id].[cb_topic]' where 'field_id' is the id of my custom 'cb_topic' field?

Doesn't sound like you need it to be field or user specific so my above topic example should be all you need.

cb_topic will get substituted with the actual value of an user's profile setting?

Yes, since assets support substitutions.

I'd appreciate a link to good documentation, if available, to stop me asking questions that are answered somewhere else.

There's no documentation for this. Feel free to post whatever questions you may have here. Unfortunately our documentation for plugins is somewhat lacking as we're a small team and just haven't found the time to pause development to focus on documentation. We'll be exploring better options on how we can generate documentation automatically or at least partially automatically to try and help with this sometime in the future.