I'm not sure if this comment is quite appropriate on this thread (as it is a borderline commercial offer and quite possibly off-topic for the OP's question), but I think your model is interesting and I'd love to know a little more.

1) Is your 5-10 hours/week offer indefinite or for some fixed period? Are you capping the number of start-ups you are working with? (This offer doesn't seem to be infinitely scalable.)

2) Do you see this role more as employee-working-for-stock or as angel-investor-contributing-design-skills-instead-of-money? Maybe another way of asking that is: are you taking direction from the founders/owners or are you offering advice/support as you see fit? Are you OK with a founder who only wants you slice up a PSD she designed or are you looking for an adviser role?

3) I think there is a lot of sense in pg's advice around giving up a small percentage of the company in exchange for a much greater chance of success, but I'm not sure it is obvious that your offer is a "great deal" for the founders as you state.

I'll accept that in some cases a good design can increase the value/success of a company by 10% or more but I think in this specific scenario (early stage start-up, possibly with long-hanging fruit design challenges):

a) A 10% improvement in the current value of the company probably isn't worth 3% of the eventual value of the company if it is successful. That is, it may be easy to take a company from $1000/mo to $1100/mo through 5-10 hours of design, but barring major oversight I think it will be a lot more difficult to take a company from $10M to $11M through graphic/web design alone.

b) On the founder side of the equation whether or not the design work will increase the value of the company more than it cost is really only half of the question. The other half is comparison shopping: is there a cheaper way to do this?

For instance, if you assume a modest $500,000 exit then 3-5% is $15,000 to $35,000, which may be a roughly competitive rate for design services (e.g., that's $60 to $100/hr if you assume 50 5 hour weeks). Obviously if you assume a $1M exit those rates double. If you assume a $5M exit those rates grow by an order of magnitude, and I hope I'm not a cheapskate in thinking that $600 to $1000/hr is a disproportionately high rate for all but the most remarkable web designers and even then is probably only appropriate for the most design sensitive web apps (whichever those are).

I may well be wrong, but I have a hard time believing that for most start-ups a bit of early stage design work really provides an additional 6-7 figures of valuation in the long run. Is that what are saying is "ridiculously easy"?

c) A moderately successful start up should be able to afford market rates for design services, and I'd guess that it is rare for great rather than merely acceptable design to be the difference between moderate success and failure. Why shouldn't a firm just limp along with an acceptable design until they can afford a great one? (And if they don't have an acceptable design yet they can certainly get one for less than 3% of the company.)

In other words, I think the major challenge to your offer isn't about whether or not design can offer a 10% improvement but more about whether or not 3% of the company is the easiest/cheapest/lowest risk way to get that improvement.