But is it possible to control opacity, so that I can have localised control over the effect of a preset?
Which preset are you looking to reduce?
Normally presets alter Brightness, Contrast, Structure, Color Filters, etc. There is no way I know of to control the overall opacity of all of those potential settings a preset may make. You can use a control point to adjust Brightness, Contrast and Structure in a selective area.
To accomplish what I think you're asking for I usually go back to see what the preset changed from base and make changes.
Perhaps easiest would be to open your file in Photoshop, create a duplicate layer then use Silver Efex Pro 2 on that layer then adjust the opacity of that layer.
If you're looking for a tool that might do what I think you're looking for and you're on a Mac, check out Tonality Pro from MacPhun. It provides layers within the tool and you can control opacity there without going to Photoshop.
Hope this helps