Det ser ikke umiddelbart ud til at man kan.
Hvis man laver en get-help på kommandoen får man kun dette tilbage.
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NAME
Mount-DiskImage
SYNTAX
Mount-DiskImage [-ImagePath] <string[]> [-StorageType <StorageType> {Unknown | ISO | VHD | VHDX}] [-Access <Access> {Unknown | ReadWrite | ReadOnly}] [-NoDriveLetter] [-CimSession <CimSession[]>] [-ThrottleLimit <int>] [-AsJob] [-PassThru] [-WhatIf] [-Confirm] [<CommonParameters>]
Mount-DiskImage -InputObject <CimInstance#MSFT_DiskImage[]> [-Access <Access> {Unknown | ReadWrite | ReadOnly}] [-NoDriveLetter] [-CimSession <CimSession[]>] [-ThrottleLimit <int>] [-AsJob] [-PassThru] [-WhatIf] [-Confirm] [<CommonParameters>]
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En google søgning giver nogen lunde sammen resultat, dog med et par methoder, til at finde ud af hvilket bogstav mountet har fået.
http://stackoverflow.com/questions/16452901/how-do-i-get-the-drive-letter-for-the-iso-i-mounted-with-mount-diskimagehttp://rcmtech.wordpress.com/2012/12/07/powershell-mounting-and-dismounting-iso-images-on-windows-server-2012-and-windows-8/