Can I use Smeargle's "Portrait" Poke-POWER if I have already used a Supporter this turn?
Yes, you are not playing a second Supporter, you are just using the Supporter's effect as the effect of the power.
Source: Undaunted FAQ (2010-09-09), PUI Rules Team (2010-09-09)

Relevant cards

Note: All applicable cards may not be listed below.
COL1_21
HGSS3_8