If I filter a teammate out of my user panel, can I still view their day plan?

No. You’ll need to add them back into your user panel to do that. Just click [number of] People Hidden in the bottom left of your Daycast window, check the names of the users you’d like to add back in, then click somewhere else in the app to collapse the filter menu and see the newly added users. Now you can navigate to their day plan as usual by clicking on their name in your user panel.