Dear Gopu,
I am afraid whether such a view, particularly from the Management's side, would be correct. Union membership is a fundamental right of employees. Becoming an office bearer of a union springs up from one's membership in that union only. Holding an office in the union cannot interfere with any employees' regular duties and responsibilities. If it is your argument that the work of the maintenance department would be more demanding, it is for the individual to arrange his union works accordingly. Even if the above view is from a fellow member of the same union, it cannot be upheld for from the point of view of effective functioning of the union, the services of its office bearers are quite essential. When the person from the maintenance department is the best-suited for a particular post in the union, you cannot leave him simply aside because of his job.
Therefore, in my opinion, as long as an individual employee/union member is otherwise qualified for any official position in the union, his job position or department in the organization cannot be a bar.
Most importantly, the employer has no locus standi in such an affair of a union.