Hello,
am I understanding you right, that you would like ONE SNMP trap per ONE dropped packet? This does not sound like a good idea because of the CPU impact it would have when dropping thousands of packets. What you can do is to monitor dropped packets by reading interface counters through SNMP as usual.
There will be no customer related information however because within an MPLS cloud a P router has no concept or knowledge about "customers". This is one of the main ideas of MPLS - separation of transportation and service (customer).
If the customer is concerned with your SLA I would recommend to have a look at SAA. It will allow you to monitor even application performance, availability and the like across the MPLS cloud. This is what usually is done, afaik. Again, you should understand what those measurements will tell (and what not).
And as long as you have configured your MPLS MTU properly there will be NO issues because of packet size across MPLS.´
Hope this helps! Please rate all posts.
Regards, Martin