As the New-OSCustomizationSpec api is from VMware.VimAutomation.Core, found a a work around to execute this command alone in 32 bit.
start-job -RunAs32 {
Add-PSSnapin VMware.VimAutomation.Core
New-OSCustomizationSpec -Name $osCustomizationSpecName ... ... ...
} | wait-job | Receive-Job
This runs the command in 32 bit environment and returns the result.