Publishing an S/Mime Certificate to the GAL with Powershell

Question

I help manage an environment where all of our users should have access to an email certificate from an external CA for the purpose of sending encrypted email. A number of these users are on Macs, and one frustrating feature of Office for Mac is that it does not feature the button that allows the user to publish their certificate to the GAL.

I've been investigating solutions for publishing to the GAL via Powershell rather than relying on Office. I've seen this topic discussed on a few web pages and have repeatedly seen people linked to this page which suggests using the following script:

$cert=New-Object System.Security.Cryptography.X509Certificates.X509Certificate2("C:\temp\cert.cer") 
$certArray = New-Object System.Collections.ArrayList
$certArray.Insert(0,$cert.GetRawCertData())
Set-Mailbox -Identity <Mailbox UPN> -UserCertificate $certArray

While this will run and not output any errors, it doesn't appear to actually do anything as far as I can tell. I worry I may be going down a blind alley and that it isn't possible to publish to the GAL other than through Outlook. Has anyone had success with a method like this or an alternate method for publishing certs to the GAL outside of Outlook?