I did use mushroom head Allen M4 fasteners to attach a repainted bridge to the mudguard. If you use nylon washers under the head of the Allen bolt, that should protect the chromium plate. The M4 x 8 fasteners are very small, and you have to look quite close to notice that they're not rivets:
MudguardBridgeM4Fasteners
MudguardBridgeM4Nuts
Personally, I hate rivets, as they make future maintenance more difficult - I liken them to nails in joinery. I attached my new ID plate with Pozidriv self-tapping screws rather than use rivets:
ForkReassembly