I don't claim to be an expert and I looked at tooltips a long time ago when they were not directly supported by PB. I'm guessing that you want the information displayed in the tooltop to be dynamic - that is, the value displayed is retrieved immediately before the tooltip is displayed. AFAIK there isn't a good way to do this with PB. I suggest you go to the SDK documentation and look up the notification code TTN_GetDispInfo that is used with WM_NOTIFY. That is the notice from the OS to the application that the tooltip is about to be displayed so that the app can set the appropriate value for display.
However, I don't think I would try to use this approach to do a database retrieval for the txt. The app will get lots of these notifications and (ignoring the error handling / notification issues associated with the retrieval) it seems like some sort of caching would be a requirement for acceptable performance. How that fits with the tooltip support for PB I can't say. Again, my experiments (assisted by a demo from Phil Salgannik) involved older versions of PB that did not have any support - PB might be intercepting some of the messages now making it difficult.