Not really.  Existing code is written with instanceCap of 0 meaning 0, which causes an effective lockdown on auto-provisioning of that config (you can still manually provision them via the web drop-down though, which could be considered a bug I suppose).

Also the existing instanceCap code you mention actually treats the null/empty configuration as I am doing by substituting Integer.MAX_VAL. 
i.e. SlaveTemplate.getInstanceCapStr() is implemented as follows:

    public String getInstanceCapStr() {
        if (instanceCap==Integer.MAX_VALUE) {
            return "";
        } else {
            return String.valueOf(instanceCap);
        }
    }