Here's my own attempt. Better solutions will be graciously accepted!
Function ToDateTimeStringMinimalSeparators(dateTime)
    ' --------------------------------------------------------------------------
    '  F O R M A T   T H E   U T C   O F F S E T
    ' --------------------------------------------------------------------------
    Dim oShell, activeTimeBias
    Set oShell = CreateObject("WScript.Shell")
    activeTimeBias = oShell.RegRead("HKEY_LOCAL_MACHINE\System\" & _
        "CurrentControlSet\Control\TimeZoneInformation\ActiveTimeBias")
    Dim sign
    sign = "-"
    If activeTimeBias < 0 Then
        sign = "+"
        ' Make it positive while we're doing calculations
        activeTimeBias = activeTimeBias * -1
    End If
    Dim atbHours, atbMins
    atbHours = Right("00" & Int(activeTimeBias / 60), 2)
    atbMins = Right("00" & (activeTimeBias Mod 60), 2)
    If atbMins = "00" Then
        atbMins = ""
    End If
    Dim utcOffset
    utcOffset = sign & atbHours & atbMins
    ' --------------------------------------------------------------------------
    '  F O R M A T   T H E   M A I N   D A T E
    ' --------------------------------------------------------------------------
    Dim dateBody
    dateBody = Right("0000" & Year(dateTime), 4) & _
        Right("00" & Month(dateTime), 2) & _
        Right("00" & Day(dateTime), 2) & _
        "T" & _
        Right("00" & Hour(dateTime), 2) & _
        Right("00" & Minute(dateTime), 2) & _
        Right("00" & Second(dateTime), 2)
    ToDateTimeStringMinimalSeparators = dateBody & utcOffset
End Function