The IRS' required information list asks for Form 1120, 1120S, 1040 or 1065 (including all statements, schedules and attachments) filed by the employer for the year under audit.

The information in Form 1120, 1120S and 1065 is limited to the employer's business and giving them to the IRS is not a problem. But Form 1040 (for a sole prop) mostly includes non-business and non-pension plan related info and that could present a problem as the examinar may go on a fishing trip through the whole return!

Can the IRS demand to look at the complete Form 1040 when auditing a pension plan? Can one provide Page 1 of 1040 and Sch C only and refuse to to provide the rest of the return?

Historically a pension plan auditor wouldn't even be trained to look for individual issues on a 1040 -- the auditor would just be looking at the deduction for the pension plan.