If your iOS device is jailbroken, you can install WeBe++.
This app can be used in Bluetooth HID mode which means that no software is required on the Mac to 'receive' the mouse input. This is what requires the jailbreak and why apps that do this in this way won't be found on the App Store. WeBe++ uses BTstack, a separate Bluetooth stack that gives more possibilities to tweak developers—BTstack will temporarily disable Apple's Bluetooth to override it into HID allowing representation of mice/keyboards, meaning no software is required on the Mac. WeBe++ supports clicking as well as scrolling, but no multi-touch gestures.
The Wi-Fi apps that are available require software to be installed on the Mac regardless of whether Wi-Fi is present or an Ad-Hoc network is created. If you can manage to get software installed on the Mac to connect to Wi-Fi mice apps, then you can create an Ad-Hoc network to connect your iOS device to. Go to the AirPort menu bar item and create a network. Apps that work over Wi-Fi will work over an Ad-Hoc network, so this may be something you could set up for future necessity.