Scanned detailed comments for a paper review
I think it really comes down to the editor. You should first ask the editor of the journal if there is any issue and then think if your handwriting is clear enough. Hoewver, please keep in mind that scanned copies cannot be copy - pasted. You could return a pdf or Word version of the paper with annotated comments, either on the side or baloons (Adobe Acrobat and Adobe Reader allow for that and I find it very useful when finalising a draft). This sounds to me like a more convenient solution that a scanned copy.
I would like to stress that, despite your good intentions, the full responsibility of presentation and language falls on the authors, not the referee. A paper is not just about content but also sound presentation and communication of ideas.