There’s more than one thousand literary agents in the US and yet I do not know of any who specialize or even handle technical writing assignments. That’s not to say there aren’t any, but I suspect they are few and far between. The reason is that there’s not a lot of financial incentive for them. As you would know, agents in any field only make money when they make a sale – and there’s far more money to be made short and long-term when they sell literary properties, such as novels or screenplays, rather than one-off freelance assignments, such as technical manuals.
I think your best bet is to apply the marketing principles outlined throughout my book. Start, by putting together a promotional pack on you and your services and then target relevant firms in industries of interest. I believe direct mail is the most effective approach – in terms of time, cost and, of course, it demonstrates your literary abilities and what you can offer the prospective client.