I work at a dealer and the answer is yes and no. Some dealers will not touch your vehicle unless they do all the services/maintenance. Some are ok with it as long as you keep maintenance records and receipts for all fluids/filters/gaskets/shims, etc. used as proof...and may or may not specify that those materials are purchased through them.Is there any clarification as to whether or not the warranty is void unless the KTM dealer does the service?
Technically unless you F'd it up it should be covered under warranty but some dealers will simply refuse to take in your motorcycle. Policies vary. Best thing to do is talk to your dealer's service manager and find out.