Figured it out, posting the answer for others who need this functionality as well:
awful.key({ modkey, "Mod1" }, "Right", function () awful.tag.incmwfact( 0.01) end),
awful.key({ modkey, "Mod1" }, "Left", function () awful.tag.incmwfact(-0.01) end),
awful.key({ modkey, "Mod1" }, "Down", function () awful.client.incwfact( 0.01) end),
awful.key({ modkey, "Mod1" }, "Up", function () awful.client.incwfact(-0.01) end),
Basically, instead of tag's incmwfact
, use the client's own incwfact
function. Also, this will only work in tiling layouts, in floating it will cause an error bubble.