Just figured this out today if still interested.
Once you create the external user request, you can query the 'Access Requests' list. Find the access request you created using the 'RequestedFor' field. Then you can look at the 'PermissionLevelRequested' field, which is a numerical value corresponding to the SharePoint security group id. Update this field with the appropriate id for the target group. Have tested and the changes are reflected on the 'Access Requests and Invitations' page in the GUI.
Be aware that there is a 'PermissionType' field as well. Generating requests using the same method above, mine was always set to 'SharePoint Group'. Would be smart to confirm yours is as well, doubt this would work with a different value in that field.
Test code below, haven't tested this in a full script yet, not sure if you would need to add a Start-Sleep to let the invitation process on the site after sending it.
Does anyone know a PowerShell Script which will assist to clone the permissions from "domain\username" to "external email address" (Eg. : email@example.com) ?
Thanks & Regards,