Last time I checked, there was no sufficiently capable SVG editor available that is both online and free.
However, I did just find
https://fixthephoto.com/uk/illustrator-online.html, which actually lets you set the object IDs that you need to make a WZ map.
It does appear to be rather clunky with how it handles pixel sizes of the document.
But I only did a very quick test to see if it was usable at all, so no clue how well it actually works for map making.
(Note that this is a commercial website, I cannot guarantee it's entirely free, and if you don't pay with money you're paying with your data.) I would nevertheless recommend just using Inkscape if you have the ability to download something, since that makes getting help and tips & tricks a lot easier.
