So are low hashrate miners having problems in your current system?
No, they have just higher variance, but it averages out itself (after 1000-10000 shares).
So if their handicap were proportional to their reward from the last round, it wouldn't come CLOSE to covering the pain caused by the shares granted to the high end traders.
Joe Blow 20ghash earns 200000 shares in round 123 in 200 minutes
Jane Blow 2ghash earns 20000 shares in round 123 in 200 minutes
Slowie mc whybother 50 mhashes earns 50 shares in round 123 in 200 minutes
Scammer mcdouchpants 100 mhashs earns 100 shares in 200 minutes
So in the next round you calculate the estimated hashes per user in the first few minutes:
joe bloe 20ghash starts off with 5000 loyalty shares (5 minute estimated share volume)
jane blow 2ghash starts off with 500 loyalty shares
slowie mc whybother ends up with 12.5 or whatever loyalty shares.
Scammer mcdouchpants gets 25 loyalty shares
So if the next round ends in only two minutes
Joe bloe would get 5000 loyalty shares + 2000 earned shares
jane blow 2ghash would get 500 loyalty shares + 200 earned shares
Slowie mc whybother ends up with 12.5 loyalty shares and somewhere around 4 earned shares on the round
Scammer mcdouchpants would end up with 25 loyalty shares and lets say he's running 20ghash and he pulls 2000 earned shares.
So scammer mcdouchpants would be hurt badly in this situation because his payoff would be only 2/7ths what it would be if there were no loyalty shares.
For anyone STAYING in your pool, their percentage of payments would stay the same (actually they'd see a ~5% increase in earnings).
Pool hoppers would see a ~62% drop in profitability.
Also... people that bail on long blocks would be penalized a bit.
 my point is that the aggregate share proportions amongst loyal users shouldn't be affected much, except because they'll have more shares than anyone that was not laboring in the previous block. If the very FIRST share recieved was the block hash, the payout would be almost identical to the payout for the previous block.
So if you are a scammer and that block was 2 hours long... you're out of luck. Because even in 5 minutes, you'd still be looking at 50% of the shares relative to your processing power for the block. In the upper area it'd make hardly any difference.
Bottom line? You get guaranteed safety from pool hopping and you have to work through one block at a decreased rate.