I’m not familiar with PSAppDeployToolkit.
And while we don’t have a built-in way to present this kind of ‘persistent message’ in BigFix itself, if it helps what I think you’re looking for is a ‘Modal Dialog’. I have some notes on it here Force Docx File to auto open on users end - #9 by JasonWalker including how to present such a dialog in VBScript that could probable be adapted to PowerShell as well.