There are currently technical limitations in gnucash that make it difficult to implement a lookup for vendors when printing checks. Additionally vendors would not be sufficient. Employee vouchers and Customer credit notes may also be paid by checks, so lookups for those would be useful as well.

While not exactly what you are requesting, gnucash 2.6.12 will have the ability to print checks from the Process Payment window for Customers, Vendors and Employees. If these people/companies have addresses specified, these will be used for printing on the check.
In addition, if you didn't print a check when you processed a customer/vendor/employee payment, you can still print a check from the transaction that was generated from that payment. Again gnucash will figure out the address from the partner information associated with that payment.

This new functionality does require the use of the business features. If you are not, there is no address information available.

Would this be sufficient to close your request ?

Since many vendors or customers have multiple locations and multiple departments, there should be room for an "Attention:" field or department field as a modifier to point to a different address.

I'm available for contracting work on features such as this one. Feel free to contact me via christian@cstimming.de - thanks!

This works for using windowed envelopes. Very handy for bill paying.In the appendix of Tutorial & Concepts reference is made to positioning "ADDRESS" for printing but, at least in 2.2.9, I can't find where the payee's address is recorded and accessed. I use PCLINUXOS.