At the NSF conference, we’re talking about cost sharing, and the fact the NSF does not allow voluntary committed cost share from institutions on proposals, except where required. However, in the case where institutions provide supplemental salary for 9 month faculty members who are requesting an additional 2 months of salary support for research activity, or similar types of activities (other examples were providing post docs to projects) be listed on the equipment and resources page.
The application should specify, not the dollar amount, but the programmatic resources committed to the project. However, the NSF will consider that a commitment of resources that you will need to report on, and it can be audited. While the project officers and NSF representatives discussed this as voluntary uncommitted cost share that would be subject to a program audit, the specific requirements ask for such detail that a dollar amount can easily be assigned to the resource.
I would talk to your Office of Sponsored Research before doing this. I understand why NSF is asking for the detail, but its as close to voluntary committed cost sharing as you can get without writing a dollar amount.