I have been working with SAP Support on this, and this is what we've come up with so far. Because sps1 for EhP8 is part of the base release, it is included in the upgrade DVD and therefore not all components that will be included are shown in the stack file from Maintenance Planner, as there are not yet additional support packs beyond this for most of these components. Ok, I can accept that. So, the upgrade should correctly update both SAP_HR and EA-HR to 608.
There is still a bug in the Add HR Packages feature, however. It should not be offering EA-HR 604 support packs as part of an upgrade to EA-HR 608. As such, currently it does not appear to be possible to include all the HR support packages in a single step with the upgrade.
The base package for EhP8, sps1, includes SAP_HR and EA-HR 608 sp21. This looks to be equivalent to sp93 for SAP_HR/EA-HR 604. So, as long as you aren't already on a higher HRSP level than 93 in the source release, it should be possible to safely upgrade to 608 without including additional HR packages, and then perform a separate support pack update to the latest HR packages after the upgrade is complete. That's a hassle, and not the way the tool is supposed to work, but it will probably be doable.
I'm currently on HRSP 92 in my source release, so I can likely make this work for me.
If a customer was on HRSP 94 or higher, this would not work, as it would result in a downgrade of their HR components during the upgrade, and thus run a very real risk of data loss.