The 21-year-old, who previously played for Scottish outfit Gala, will act as front-row cover with Alex Allan nursing a leg injury.
"Ewan has fitted in well with the squad and has coped with the increased physicality of professional rugby training," Scott told the club's official website.
"It's a fantastic learning opportunity for him to improve as a player by testing himself against some of the top front-rows in the country in training, as well as push for a place in our squad for the remaining three games of the season.
"Scotland's domestic divisions are the bedrock of rugby in this country so hopefully this call-up will also show other players in the amateur game that, if they work hard on their conditioning and are performing consistently for their club, then opportunities for Edinburgh Rugby can come their way."
Should McQuillin impress, it is understood that he will be handed a long-term deal.