If two guides are selected, the "Distance" option can be selected by right-clicking. In an extra window, the first or second selected guideline is set as the origin, as well as the axis on which the distance is to be manipulated and the value to be manipulated. It would also be useful if you could mirror guides along other guides by simply right-clicking.
Leon, it feels like the exact thing native Transform panel can do. If you select two guides (let’s say vertical ones), the panel displays the distance between them as 'width', and the reference point block allows you to choose which side you want to stick to when changing it.
It is not that accessible as you want in the request, no context menu (there is a separate request to have it generally customizable, https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/32692501-right-click-menu-enhancements-cut-copy-paste-l), and you can’ use a hotkey to quickly focus the Width and Height fields in Transform sections of Properties/Control/Transform panels (there is another request to have it, https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/34613497-transform-shortcuts), and you have to set the reference point by clicking, instead of hotkey-ing one (there is a request for this too, https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/32403805-make-setting-reference-point-hotkeyable) but it’s doable.
Finally, you can also flip with the reference point set up, with Flip Horizontally / Vertically commands from the Transform panel’s menu (which can be put into an action, but not assigned to a direct hotkey).
How does it differ from the task you have at hand? Please comment back.