I could see an experience table that sets the number of your hit dice you can use to calculate your hp, attack bonus, and saves, and then a separate table that lists how much xp it takes to advance into each level of a class.

As long as you avoid front-loading class feature multiclassing gains, you could be a ftr3/wiz4/clr/2/rg5 and you'd be fairly balanced against say... a straight 9th level class as long as it works out on the table. This would work better if you define fractional progressions for attack and saves for each class and then round down the total.

In this example, you'd use the ftr, cl, and 4 rg hit dice for your hp (assuming ftr>clr>rg>wiz), and the 9 best class levels to determine each of your saves and attack bonus.