As far as I can tell Phillip never paticipated personally in the Olympic Games, however after he was king for several years and recognized as a power to be dealt with, he did enter several of his horses in the races, one of which won his heat.
As for Demosthenes' Phillipics, while he was exhorting all the Greeks to resist Phillip, he was after all a citizen of Athens, which would indeed give him reason to be "Athens-centric".
Further, it's absurd to say that only Demosthenes considered the Macedonians barbarians:
Thanks for the links to some good articles.
Macedonians could participate in the Olympics centuries before King Philip was born. Historic accounts state King Philip participated and did win three events at three different Olympics: in 356 BCE the year Alexander was born, during the 106th Olympics he won the race riding his horse; 352 BCE during the 107th Olympicshe won the four-horse chariot race; and in 348 BCE during the the 108th Olympics he was the winner of the two colt chariot.
http://www.museum.upenn.edu/new/olympics/journal2.shtml
As far as Demosthenes' Philippics goes he was hardly exhorting all the Greeks to resist Phillip, quite the opposite actually. He even reminds the Athenians that they fought with others against the Lacedaemonians(Spartans) for Greece so why don't they fight against Philip for Greece now, in other words Demosthenes makes their fight for Greece against Philip equivalent to their fight for Greece against Spartan, and Sparta is never called not Greek. In his condescending speech he makes it very clear that Athens has to fight by herself and not rely on anyone else or she will be taken down. He makes it very clear in his speech that there were Greeks who would side with Philip over Athens in this war. In other words a typical Hellenes vs Hellenes political war that we see often during the ancient Greek world.
"And let not this escape you, Athenians, that you have now the choice, whether you shall fight there, or he in your country. If Olynthus hold out, you will fight there and distress his dominions, enjoying your own home in peace. If Philip take that city, who shall then prevent his marching here? Thebans? I wish it be not too harsh to say, they will be ready to join in the invasion. Phocians? who can not defend their own country without your assistance. Or some other ally? But, good sir, he will not desire!"
http://www.gutenberg.org/dirs/etext04/lnppp10.txt
Another thing that apro stated which is spot on is the fact that Demosthenes' beef was with King Philip alone, he never stated anything about the Macedonians in general being 'barbarians' and in his second speech he makes himself clear that he does not believe the Macedonian subjects are to be blamed for their King's acts, in other words they are just as much "victims" of their King in Demosthenes' eyes:
"Think not, Athenians, that Philip and his subjects have the same likings. He desires glory, makes that his passion, is ready for any consequence of adventure and peril, preferring to a life of safety the honor of achieving what no Macedonian king ever did before. They have no share in the glorious result; ever harassed by these excursions up and down, they suffer and toil incessantly, allowed no leisure for their employments or private concerns, unable even to dispose of their hard earnings, the markets of the country being closed on account of the war. By this then may easily be seen, how the Macedonians in general are disposed to Philip. His mercenaries and guards, indeed, have the reputation of admirable and well-trained soldiers, but, as I heard from one who had been in the country, a man incapable of falsehood, they are no better than others. For if there be any among them experienced in battles and campaigns, Philip is jealous of such men and drives them away, he says, wishing to keep the glory of all actions to himself; his jealousy (among other failings) being excessive. Or if any man be generally good and virtuous, unable to bear Philip's daily intemperances, drunkenness, and indecencies."
http://www.gutenberg.org/dirs/etext04/lnppp10.txt