I think that you might be partially wrong. I know of two different cases. The first where there is a priest that always attends the midnight Xmas Mass and he sits in the front pew, he is not a member of the clergy in that parish. During communion the pastor of the parish extends he ciborium to him and he self communicates. The second happened to me where one of the parish priests was attending Mass and seating in the first pew, handicapped seats, because he could not stand. I was the EMHC and when I approached him with the ciborium he self communicated.
Although this was done, I would argue it was still incorrect.
Firstly, we must recognize that the tradition of priest attending Mass
en choro is no longer done frequently. When a priest attends Mass
en choro, he would sit in the sanctuary, and be vested in only a cassock and surplice. The priest would not participate as a concelebrant. The priest would don a stole for the reception of Holy Communion, and would be used to distribute the Blessed Sacrament before any EMHC could be used. Immediately after he is done handling the Blessed Sacrament, the priest would doff his stole.
This practice is not as common as it once was, since priestly concelebration has been restored. However, some priests still opt to attend Mass in choir for various reasons (they have already celebrated several Masses that day, they are in a state of mortal sin, or they are leaving early). Now, bishops are really the only clerics frequently seen in choir dress, since they cannot concelebrate when a priest is the primary celebrant.
Most liturgical documents separate those who participate in the Mass into two groups:
1) celebrants (and concelebrants),
2) the faithful. The documents specify that only celebrants and concelebrants may self-communicate. While we may initially think that a priest could not be counted among the faithful, we should recognize that he is certainly
not participating as a celebrant or concelebrant. I would argue, then, that the documents indicate he should not self-communicate.
That’s my take on it.