This is quite nice and general approach:

SetAttributes[mySequence, HoldAllComplete]; mySequence[args__] := RawBoxes[ MakeBoxes[{args}][[1, 2]] ]; y = 7; Hold[{1, x, 2, x, 3}] /. x :> RuleCondition @ mySequence[y, 1+2] 

 Hold[{1, y, 1 + 2, 2, y, 1 + 2, 3}] 

I though it is nice untill I had to add reparse function :)

SetAttributes[mySequence, HoldAllComplete]; mySequence[args__] := RawBoxes[ MakeBoxes[{args}][[1, 2]] ]; reparse = ToExpression @* FrontEndExecute @* FrontEnd`ReparseBoxStructurePacket @* ToBoxes y = 7; Hold[{1, x, 2, x, 3}] /. x :> RuleCondition @ mySequence[y, 1+2] //reparse 

 Hold[{1, y, 1 + 2, 2, y, 1 + 2, 3}]