Resolved:
Instead of using this as a custom measure:
[Laptop AssetID].CurrentHierarchyMember.Get('Total RAM (MB)')
Use this instead:
/* find member in the Laptop AssetID dimension
that matches Laptop name in issue property */
StrToMember(
"[Laptop AssetID].[" ||
[Measures].[Issue Laptop AssetID]
|| "]"
--read property Total RAM for member in "Laptop AssetID" dimension
).Get('Serial Number')