@Peter Gulia, having a cap on the percentage of a contribution that can be invested in a specific investment is used by some plans to limit investments in:
publicly traded stock of the employer,
self-directed brokerage accounts (particularly when there are few restrictions on permissible investments within the SDBA),
investments in that are not easily tradable like gold bullion or real estate, and
investments where the plan fiduciaries are concerned about the volatility of the investment.
Most recordkeepers can support this type of limit. Note, though, that recordkeepers may not support automatic re-balancing when the value of these investments exceed a specified percentage of the value of a participant's overall plan account.