You wouldn't be the first one to do it. A local mechanic has a WTB Horizon, with mudguard. I don't know if he did anything to the chain stays.
The Omnium International Owners group on FB is good for that sort of queries.
Nice one, was not aware of that group - Much better for technical stuff then the official omnium one. Had a scroll through and apparently 650x47 horizon just about clears with no mods.
Now have more important problem to deal with of choosing a colour. ¯_(ツ)_/¯
You wouldn't be the first one to do it. A local mechanic has a WTB Horizon, with mudguard. I don't know if he did anything to the chain stays.
The Omnium International Owners group on FB is good for that sort of queries.