Thank you for your inquiry.
We, 2019 OnStage TC, have discussed it and concluded. Unfortunately, we cannot allow for any Advanced teams to use mats (and similar things). There are two reasons for it.
It is stated clearly in Preface as below:
The rules are mostly the same for OnStage Preliminary and OnStage Advanced. However, only teams in OnStage Preliminary are allowed to use markings and/or mats on the floor to enable line following. OnStage Advanced teams are expected to use more advanced technologies.
Also, there is additional advice for designing and constructing robots in the rule 1.5.1:
1.5.1. While floor joints will be made to be as smooth as possible, robots must be prepared for irregularities of up to 5 mm in the floor surface. Whilst every effort will be made to make the stage flat, this may not be possible, and teams should be prepared to cope with this uncertainty.
Advanced teams are expected to design and construct their own robots which can cope with some irregularities. Additionally, we think that it would be better for judges to evaluate all robotics performances in the same environment.
Thank you for your understanding.