"merge field" is simply a display format. It has no effect on what text is stored in that field. If the text field stores a valid URL, regardless of whether you place the field on you layout or place it as a merge field, you should be able to setup the web viewer to use that field as its URL for displaying a web page.
You can find an example of this in my Known Bugs List database. On the Bug Reports layout, there's a portal on the Web tab that uses the text in a field named FMP Bugs::Link. In the Web Viewer set up dialog, I simply used the Specify button to select this field for the Web Address.
I use that field by copy/pasting the URL from the top of my Web Browser into that text field to link a bug report record to a specific web page in Report an Issue where the bug was reported and confirmed.