Hi Jamie,
Yes, this is possible - anything can be added to plots of bamm data. For example, you can add a very simple timescale with the axisPhylo() function from the ape package.
library(BAMMtools)
library(ape)
data(whales, events.whales)
ed <- getEventData(whales, events.whales, burnin=0.25, nsamples=500)
plot(ed, lwd = 3, spex = "s")
axisPhylo()
There are other R packages that provide more complex timescales, including for example geological periods, etc. Those could be added as well.
-Pascal