Hm, tough stuff, Glicko certainly doesn't have much in terms of wiggle room.
After reading through the paper, I think the best option here would be to declare each poker round a Glicko "period". Then we could view the weighting as players having played N times against each other - so in my approximate "linear" mode, the heads-up would be equivalent to 9 games of 1st vs 2nd, whereas finishing last would be equivalent to only 1 game against every player at the table.