Custom Install that needs to write to HKCU

Scripted Actions aren't generally a good solution for user registry changes because Scripted Actions run as SYSTEM.

You could do this as a scripted action that creates a scheduled task that runs on user logon.

I'm no PowerShell wizard, but something like this should do what you're wanting.
Do you think the customer would be willing to do that?

$taskName = "ApplyRegistryChangesOnLogon"
$taskDescription = "This task applies registry changes on user logon."
$scriptBlock = @"
    # Apply registry changes
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\{20770A70-7470-49B6-B565-1FA1398189D5}' -Name 'PreferredClr' -Value 'v4.0.30319' -Type String -SID $env:USERPROFILE
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\{219EEE1C-85C7-49F9-AC95-CB3BA073FD77}' -Name 'PreferredClr' -Value 'v4.0.30319' -Type String -SID $env:USERPROFILE
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\{3ED92CF8-2820-4570-A4A8-67DAA4E07E73}' -Name 'PreferredClr' -Value 'v4.0.30319' -Type String -SID $env:USERPROFILE
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\' -Name 'C:\ProgramData\UpSlide\Binaries\Finance3point1.UpSlide.Excel.vsto' -Value '{20770A70-7470-49B6-B565-1FA1398189D5}' -Type String -SID $env:USERPROFILE
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\' -Name 'C:\ProgramData\UpSlide\Binaries\Finance3point1.UpSlide.PowerPoint.vsto' -Value '{219EEE1C-85C7-49F9-AC95-CB3BA073FD77}' -Type String -SID $env:USERPROFILE
    Set-RegistryKey -Key 'HKCU\Software\Microsoft\VSTO\SolutionMetadata\' -Name 'C:\ProgramData\UpSlide\Binaries\Finance3point1.UpSlide.Word.vsto' -Value '{3ED92CF8-2820-4570-A4A8-67DAA4E07E73}' -Type String -SID $env:USERPROFILE
"@

# Create a Scheduled Task to run the script on user logon
$action = New-ScheduledTaskAction -Execute "PowerShell.exe" -Argument "-Command $scriptBlock"
$trigger = New-ScheduledTaskTrigger -AtLogon
$principal = New-ScheduledTaskPrincipal -UserId "$env:USERNAME" -LogonType Interactive

# Register the scheduled task
Register-ScheduledTask -Action $action -Principal $principal -Trigger $trigger -TaskName $taskName -Description $taskDescription

Maybe PSADT and Active Setup?

Great idea, Ryan!
Vince Thompson, is your customer open to utilizing 3rd party tools?

Dave Stephenson Yeah they would be, for now I have done a scheduled task.  Thanks everyone for the suggestions.