UPDATE: I found something of potential relevance:
They've got a similar example:
CREATE TABLE DocumentStore (
DocumentID INT IDENTITY PRIMARY KEY,
Document VARBINARY (MAX) FILESTREAM NULL,
DocGUID UNIQUEIDENTIFIER NOT NULL ROWGUIDCOL
UNIQUE DEFAULT NEWID () ON Data_FG1)
ON DocPartScheme (DocumentID)
With a remark:
Notice that to use the DocumentID column as the partitioning column, the underlying nonclustered index that enforces the UNIQUE constraint on the DocGUID must be explicitly placed on a filegroup so that the DocumentID column can be the partitioning column. This means that partition switching is only possible if the UNIQUE constraints are disabled before performing the partition switch, as they are unaligned indexes, and then re-enabled afterwards.
I suppose I'll then leave ROWGUID be as it is, a UNIQUE nonclustered index.