>If you add a specific IP route on the HQ MX that points to the next hop on the internet link
Good thought @cmr but you can not add a static route via a WAN interface - only a VLAN interface. However you are going in the correct direction.
@MarkT this is not easy to do in Meraki land. What you basically have to do is have two MX at the HQ (or an MX and another firewall).
One MX runs in VPN concentrator mode, and is what all your AutoVPN connections terminate on, and it sits behind the next MX/firewall. The other MX (or firewall) provides the actual connection to the Internet. Now we use @cmr 's idea. The VPN concentrator will allow the static route to now be added pointing to the other firewall (which is also its default route). You can then publish this static route into AutoVPN.