Yes you can hold valves up with pistons. Be extra careful that the piston for that hole is at TDC before removing the spring.
This is more practical in a diesel because the combustion chamber is microscopic in size. So, with the piston up the valve is almost touching it anyway.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
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Over 800,000 miles in