Ok, this R code worked, thanks for pointing me in the right direction:
library(plotly)
mat <- matrix(1:100,10,10);
plot_ly(z=mat, type="heatmap", customdata=apply(mat,1, as.list), hovertemplate="%{customdata}<extra></extra>")
The problem was realizing that 1) it was working 2) I just needed to convert the data into the correct format.
It would be nice if I (or other people) knew how to get some sort of debug output or something.