IMHO, semps would not survive you placing a board on them,let alone walking on the board. I think you should plan on maintenance channels or rows between the semp plants. Perhaps on offering by "Steppables"such as a creeping mint or thyme might work between the semps.
Semps seem to be susceptible, at least initially, to "colony collapse disorder" (another name for massive blooming). I think this tendency would decline over time in an established colony as generational drift would eventually occur.
In an earlier post on another thread, I described the tendency of offsets to bloom simultaneously with parent flowering when attached to the parent. I think this is more likely to occur with types that have thick stolons to the offsets rather then types with thin stolons. I think a chemical signal diffuses through the thick stolon to the offset, triggering the flowering. As guard against this and as a propagation technique, you may want to take a pair of scissors and snip the offsets off of the stolon near the base of the offset. On thicker, healthier stolons,that may trigger the production of additional offsets while protecting existing offsets.
