Okay, it’s actually very easy, just adding these four lines into reels.init = function():
params:add_control("play", "Play", controlspec.new(0, 1, 'lin', 0, 0, ""))
params:set_action("play", function(x) reels:key(2, 1) end)
params:add_control("rec", "REC", controlspec.new(0, 1, 'lin', 0, 0, ""))
params:set_action("rec", function(x) reels:key(3, 1) end)
So this acts exactly like I would press the key 1 (play) or 2 (rec).