Online Tool for Cran checks

There is now R-Hub (github page). Made possible by the R-Consortium.

r-hub is a multi-platform build and check service for R packages. This packages uses the r-hub API to connect to r-hub and start package checks on various architectures.

No. There is currently no publicly available cross-platform checker available for R (with the possible exception of R-forge, which tends to be a bit unreliable)