I typically build a [Record ID Display]. This is a formula-text field which I use to replace the default setting of [Record ID#] as the record title in a form. I can then customize this to include key information about the record; it's parent (if there is one) and perhaps a status (if that applies). I always add the [Record ID#] on the end, so that as an Admin I have visibility to the record information when troubleshooting user issues.
Then go into the table-settings, click on Advanced Settings and change the Record Title to the formula field ([Record ID Display]. I always make this formula field non-reportable so that users do not use it in their reports.
I do not know of a solution to display custom-text that never moves from the Top of the Page as you scroll down. Even if you use TABS to control how much data the user sees in each one; if you have to scroll within a TAB down below the header of the record; you will still lose whatever you put at the top of the form. The Record Title however, is always visible.