Don't see one, but if no one has one, could perhaps start with this 28-HTSSOP package **broken link removed** and modify it to 20-HTSSOP.
Sparkfun has a tutorial on making your own library parts here
NP, guessing you were looking through the devices and not seeing the packages listed below? Noticed that the 20-HTSSOP package isn't attached to any of the devices, so you have to make one and can't just use one as a "proxy" part.
Yes, that is a shame. I try to add the package alone and Eagle says "Can't back annotate please add in the schematic". Maybe there is a device in there which is hidden, I will look again. The sparkfun library looks quite instructive so I can have a look at that too. Having the package should help make the device and speed things along, thanks.