I figured out how to add function support into the datasource (from the Method class). SO I added in support for some basic string, numeric, and date function that I find most useful/common.

One of which is the to_char function. So, create a new query from the data source, adding a new field:

to_char(participant_id) participant_id_hidden

that returns the value without the '.0' at the end of it. Customize the view of this query to not display the new field and save as the default. In the metadata for the original field (participant_id), specify a format mask of # and in the URL (this also remove the '.0' from the display), and so that I can link to the participant view from this query (since the original format with .0 results in a no data found), have the URL like:

/labkey/study/externalData/test1/participant.view?participantId=${participant_id_hidden}