I don’t believe you can compel owners to be members of the EC for a number of reasons, one being that the first step under the legislation is to be nominated, either the owner nominating themselves or someone else nominating them. In the latter case, the person being nominated can simply refuse the nomination.
Setting that aside, providing that everyone is a member of the EC could come back to bite you. For example if you have a scheme of 6 owners and only two people are willing to be on the EC, and the 4 other owners are somehow considered to be part of the EC, then the size of the EC would be 6. If the EC wanted to make a decision, you would need a quorum of 3 or more for any decisions to be made by the EC. If the 4 other owners refused to co-operate, you would not get that quorum.