I'm not an expert on Hyperlynx, but I'm pretty sure there's no 1GHz minimum frequency requirement. It's EDGES that are of primary concern, not frequency. Although you are dealing with analog signals, if you believe your problem is due to the fast rise time, then Hyperlynx will still give you a good idea of how the layout is affecting your signal; an edge is an edge, regardless of whether it's "digital" or "analog". Ultimately we're actually concerned with the analog behavior of digital signals anyway..