query multiple posts by id using a string parameter, not array()

You could convert the string to an array using the php explode function. So if you used a comma as your delimiter, you could pass the string '153,154,87' and that would and then use the explode function to convert it to

array(
[0]=>'153'
[1]=>'154'
[2]=>'87'
)

If you want to keep the p= you could use str_replace etc.