Riker was the first Starfleet officer (and likely the first Federation member) to serve on a Klingon ship.
RIKER: I don't recall hearing of a Federation officer serving ever on a Klingon vessel.
PICARD: No, no, neither have I.
(From "A Matter of Honor".)
How sweeping this statement is depends on how we read Riker's sentence. It could just mean that no Starfleet officer has ever served on a Klingon ship prior to Riker's exchange. Or it could mean that there has never been a Klingon officer of Federation origin.
There is good reason to believe that the latter is true. Consider the following:
Training to become a member of the Klingon Defence Force is demanding (and likely life-threatening), and requires basing oneself on Qo'noS or at a suitably-equipped Klingon colony.
The formal end of hostilities between the Federation and the Klingon Empire does not mean that members of the Klingon military would trust Federation members enough to serve amongst their ranks. The overall attitude would likely be that only Klingon warriors should be entrusted with the defence of the Empire. Most Klingons would also be skeptical that a Federation member would willingly die for the Empire.
Even amongst Klingons, there is discrimination about who can enter the Defence Force. General (later Chancellor) Martok, who led the Empire's forces during the Dominion War, recounted that he faced considerable discrimination earlier in his career because of his province of origin (DS9 "Once More Unto the Breach").
Klingons would likely view the combat skills of most Federation species with disdain and be skeptical about their potential for victory in battle. Federation members are seen almost universally by the Klingons as being overly concerned with peace and luxury rather than with battle and glory. The alliance between the Empire and the Federation is not based on cultural ties or common attitudes — that is for certain. Also, Federation members would be seen as lacking the requisite sense of "honour".
It's worth mentioning that food on Klingon vessels, some of which is served live, would be unpalatable to many Federation species.
These factors make it extremely unlikely for any Federation member to have served in the KDF under normal circumstances prior to Riker on the Pagh.
Further examples: Worf and Alexander
One might go so far as to include Worf and his son Alexander as examples, given their Federation citizenship and upbringing. At the end of Season 4 of TNG ("Redemption" Part 1), Worf renounced his Starfleet commission in order to serve briefly in the KDF during the Klingon Civil War. In the final season of TNG ("Firstborn"), we meet an Alexander from the future who has gone on to serve in the KDF. In DS9, we see a younger Alexander enter the KDF ("Sons and Daughters"):
MARTOK: Then why has he joined the Klingon Defence Forces?
WORF: I do not know. I have not spoken with him.
Given that Worf and Alexander are ethnic Klingons (part human in Alexander's case), I initially interpreted them as being outside the scope of your question, but given the comments below your question, it seems that their Federation citizenship renders these examples admissible.
So we have at least three examples of Federation citizens serving in the Klingon Defence Force: Riker (human), Worf (Klingon, raised on Earth), and Alexander (one-quarter human, raised in the Federation). Given the arguments above, they are probably the only canon examples of Federation citizens serving in the KDF up to and including the late 24th Century.