I can't see how you can do this directly. There is a way to do this indirectly with priority icons but only if the maps have been appropriately edited in the first place, which I guess defeats the purpose.
If (hypothetically) you were starting from scratch you could use the latest MAP add-in with the Extended Properties feature to do this. This allows you to turn priorities into numeric properties which can then be searched for in the global search.
First you use the Extended Property Feature to add the priority as a topic property [Priority (#TPy)]. Obviously the value for the priority 1 icon is 1, which is fairly useless to search for.
However, you can use a simple formula to add a large number to this topic property, creating a new topic property with a hopefully unique value - for example, [Top Priority]=[Priority (#TPy]+1000000. the [Top Priority] property would then have the value 1000001, which when searched for (with the search confined to Topic property values) is more likely to show just the relevant topics.
I suspect this isn't that useful to you but it might be helpful for someone, say, creating a suite of maps from scratch which they know they will need to search globally based on priority.
True, though MAP is a cheaper option. I concede however that my approach isn't the most elegant and has to be implemented when you set the map up, but it does work.
I've also realised there is a simpler approach which doesn't require MAP - create a Priority tag group with tags like "Pty1", "Pty2", etc, to be added to the relevant tasks. While again it would be best to start from scratch with this method, it has the advantage that these tags can be applied retrospectively, albeit only map-by-map, by filtering each map by priority and adding the relevant tag.
More broadly your question highlights the need for an extra tickbox in the Define search dialogue box for Priority. It's strange that you can search globally for values in tags but not priority.