I think the type-check is a good option. Maybe it should be able to be specified at the <accessor/> level for homogenous data sets so that it need not be redundant on each param. I also think the optional offset attribute is a good idea. Type-checking on dummies would be even more flexible with optional offsets because you could just check those offsets that matter. For example:What do you think about the type-check?
<param type="float" offset="2"/>
<param/> elements without a 'name' attribute would be type-checking dummies.
The offset should just be assumed to start at zero if omitted && increment for each new <param/> element encountered in document order.