I think transaction volume is not a good metric, it can be gamed far too easily.
And an absolute number does cause the existing supply to approach infinity.
Perhaps a percentage, but just lower? 1% is so high it really feels dirty! :p
What do you think about something like 0.5% or even 0.25%?
Expanding your point that the amount of gold is finite ... I would think we might be able to use variables like our cumulative difficulty. To me, that means overall demand for the coin. Compared to gold, there is no real limit to the amount of MRO that can be produced.
Maybe taking that variable, as part of total demand, could be used to set up a differential (applied over a certain time period ..week,month,day,or year) that we can put toward coming up with a targeted emission/inflation rate (per week,month, day or year). When demand is (edit: accelerating to a high) high, we set the inflation/supply higher and when it is low we can set those numbers lower. as far as the actual rate I'd like to hope for something lower than 1% ... but maybe a low limit of .1% and a high limit of 2%?
I'll try to think of real world things what we could use to further define the bounds, but making any particular choice (edit: probably 1%, hopefully lower) wouldn't be a bad place to start. Right now while we're producing so many coins I wouldn't want any more inflation honestly, so i hope it would be something that kicks in in 4-8 years.