I wouldn't attempt that in situ for a number of reasons. First of all, there will be distortion as the slider heats up and cools down. It is possible that this will cause the bushes to jam. Secondly, it would be awkward and you'll inevitably compromise the quality of the weld. My preference would be to fabricate the lug on a curved support, like the Gold Star mudguard mount and braze it on. Even though brazing isn't quite as strong, if the contact patch is large enough, the fix should be fine. Brazing is less likely to distort the slider where the bushes go past. I wouldn't trust myself to weld it but a professional welder might see it as quite an easy job.