Yes, it's possible. There are several ways you could do this, and it depends on your use of the model. I'm going to assume you want to completely automate everything.
I would do this with two models and runs.
In the first model, add a calculated field KEY that is a character field. The formula should be "KEY". Then add a summary. the KEY field is your key field and the mesaure is MAX(NumbericColumn). Export the summary to FirstRun.xls Save your model and project.
The second model takes your report as input. Add another calculated field KEY with the same value. Then add an external lookup to FirstRun.xls, using KEY as the common key and importing Max(NumericCOlumn). The filter of the second pass would be ImportedMax(NumericColumn)=NumericColumn. You can hide the KEY field and use Monarch as you like in the second model.
Other options that I know involve changing the filter manually based on the summary in the first pass, or using Access/Excel instead of the summary/external lookup trick.
OK, well, this is exactly where I was going, right down to the "Key" field. Joey, I swear that you were looking over my shoulder as I was typing a draft response in Notepad. Have you planted a webcam in my office?
The only thing I would add is that this can all be done in one model/project. You just need to refresh the lookups after exporting the summary, if you're doing it manually.
Of course, all of this is still contingent upon dentalexpert having Monarch Pro. Edit: OK, that's been answered now.