Is there a keyboard shortcut that I could use to set a task priority in Reminders.app on a Mac?
I'd recommend using Keyboard Maestro for just such an operation. This is perfect for a KM macro. 

EDIT: I tested the following and it works well.

Create the following AppleScript:

tell application "System Events"
    display dialog "New Reminder…" default answer ""
    set reminTitle to text returned of result
    set thePriority to 13
    set lastChars to reverse of characters -3 through -1 of reminTitle
    log lastChars
    set i to 0
    set j to 0
    repeat 3 times
        set i to i + 1
        if item i of lastChars is "!" then
            set thePriority to thePriority - 4
            set j to j + 1
        end if
    end repeat
    set reminTitle to text 1 through (-(j + 1)) of reminTitle
    if text -1 through -1 of reminTitle is " " then
        set reminTitle to text 1 through -2 of reminTitle
    end if
    tell application "Reminders"
        set newremin to make new reminder
        set name of newremin to reminTitle
        set priority of newremin to thePriority
    end tell
end tell


Then assign the AppleScript to a Keyboard Maestro shortcut.

When you run the macro, you will be prompted to enter the reminder. Use one exclamation point to set "Low" priority in Reminders, use two to set "Medium", and three for "High". Using no exclamation points will set to priority "None". Note that using this does eliminate Reminders' ability to parse due dates.









The resulting entries into Reminders.app will be...